Optimize join query involves 5 tables

EXPLAIN SELECT 
  a.year,
  a.month,
  e.large_classify_eng,
  e.middle_classify_eng,
  g.bu_detail_order,
  g.bu_detail_enterprise_name_eng,
  a.service_id,
  SUM(a.money_amount) AS money_amount,
  '04_Actual' AS b_f_a 
FROM
  t2_actual_summary_sheet_db a,
  t2_effective_bu_detail_mapping d,
  t2_bu_detail_master e,
  t2_effective_enterprise_mapping f,
  t2_enterprise_master g 
WHERE a.year = d.year 
  AND a.month = d.month 
  AND a.bu_detail_large_classify = d.bu_detail_large_classify 
  AND a.bu_detail_middle_classify = d.bu_detail_middle_classify 
  AND d.bu_detail_axis_3 = e.bu_detail_code 
  AND a.year = f.year 
  AND a.month = f.month 
  AND a.profit_center = f.profit_center 
  AND f.bu_detail_enterprise_code = g.bu_detail_enterprise_code 
  AND component_flag IS NULL 
  AND (
    a.year > 2014 
    OR (a.year = 2014 
      AND a.month >= 9)
  ) 
  AND (
    a.year < 2014 
    OR (a.year = 2014 
      AND a.month <= 10)
  ) 
GROUP BY a.year,
  a.month,
  e.large_classify,
  e.middle_classify,
  g.bu_detail_order,
  g.bu_detail_enterprise_name_eng,
  a.service_id 
ORDER BY a.year,
  a.month;

Below are table structure:

CREATE TABLE `t2_actual_summary_sheet_db` (
  `year` smallint(4) DEFAULT NULL,
  `month` tinyint(2) DEFAULT NULL,
  `account_order` int(11) DEFAULT NULL,
  `profit_center_order` int(11) DEFAULT NULL,
  `service_division` varchar(50) DEFAULT NULL,
  `expense_division` varchar(50) DEFAULT NULL,
  `charge_department` varchar(100) DEFAULT NULL,
  `logic` varchar(100) DEFAULT NULL,
  `status` varchar(50) DEFAULT NULL,
  `service_id` varchar(50) DEFAULT NULL,
  `service_name` varchar(100) DEFAULT NULL,
  `bu_detail_item` varchar(50) DEFAULT NULL,
  `account_1` varchar(100) DEFAULT NULL,
  `account_2` varchar(100) DEFAULT NULL,
  `account_3` varchar(50) DEFAULT NULL,
  `bu_detail_enclosed` varchar(100) DEFAULT NULL,
  `enterprise_1` varchar(100) DEFAULT NULL,
  `enterprise_2` varchar(100) DEFAULT NULL,
  `profit_center` varchar(50) DEFAULT NULL,
  `profit_center_name` varchar(50) DEFAULT NULL,
  `money_amount` double DEFAULT NULL,
  `bu_detail_large_classify` varchar(50) DEFAULT NULL,
  `bu_detail_middle_classify` varchar(50) DEFAULT NULL,
  `enterprise_rating` varchar(50) DEFAULT NULL,
  `component_flag` varchar(50) DEFAULT NULL,
  `facility_flag` varchar(50) DEFAULT NULL,
  `international_flag` varchar(50) DEFAULT NULL,
  `graduates_flag` varchar(50) DEFAULT NULL,
  `spare_flag_1` varchar(50) DEFAULT NULL,
  `spare_flag_2` varchar(50) DEFAULT NULL,
  `spare_flag_3` varchar(50) DEFAULT NULL,
  KEY `idx1_effective_summary_sheet_db` (`year`,`month`),
  KEY `idx2_effective_summary_sheet_db` (`month`,`money_amount`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4

---

CREATE TABLE `t2_bu_detail_master` (
  `bu_detail_code` varchar(50) NOT NULL DEFAULT '',
  `large_classify` varchar(50) DEFAULT NULL,
  `middle_classify` varchar(50) DEFAULT NULL,
  `large_classify_eng` varchar(100) DEFAULT NULL,
  `middle_classify_eng` varchar(100) DEFAULT NULL,
  `axis_1` varchar(50) DEFAULT NULL,
  `axis_2` varchar(50) DEFAULT NULL,
  `axis_3` varchar(50) DEFAULT NULL,
  `axis_4` varchar(50) DEFAULT NULL,
  `axis_5` varchar(50) DEFAULT NULL,
  KEY `idx1_bu_detail_master` (`bu_detail_code`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

---

CREATE TABLE `t2_effective_bu_detail_mapping` (
  `year` smallint(4) NOT NULL,
  `month` tinyint(2) NOT NULL,
  `bu_detail_large_classify` varchar(50) NOT NULL DEFAULT '',
  `bu_detail_middle_classify` varchar(50) NOT NULL DEFAULT '',
  `bu_detail_axis_1` varchar(50) DEFAULT NULL,
  `bu_detail_axis_2` varchar(50) DEFAULT NULL,
  `bu_detail_axis_3` varchar(50) DEFAULT NULL,
  `bu_detail_axis_4` varchar(50) DEFAULT NULL,
  `bu_detail_axis_5` varchar(50) DEFAULT NULL,
  KEY `idx1_effective_bu_detail_mapping` (`year`,`month`,`bu_detail_large_classify`,`bu_detail_middle_classify`),
  KEY `idx2_effective_bu_detail_mapping` (`year`,`month`,`bu_detail_large_classify`,`bu_detail_middle_classify`,`bu_detail_axis_1`),
  KEY `idx3_effective_bu_detail_mapping` (`year`,`month`,`bu_detail_large_classify`,`bu_detail_middle_classify`,`bu_detail_axis_2`),
  KEY `idx4_effective_bu_detail_mapping` (`year`,`month`,`bu_detail_large_classify`,`bu_detail_middle_classify`,`bu_detail_axis_3`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

---

CREATE TABLE `t2_effective_enterprise_mapping` (
  `year` smallint(4) NOT NULL,
  `month` tinyint(2) NOT NULL,
  `profit_center` varchar(50) NOT NULL DEFAULT '',
  `bu_detail_enterprise_code` varchar(50) DEFAULT NULL,
  `subsidiary_cost_existence` varchar(50) DEFAULT NULL,
  `enterprise_rating` varchar(50) DEFAULT NULL,
  KEY `idx1_effective_enterprise_mapping` (`year`,`month`,`profit_center`),
  KEY `idx2_effective_enterprise_mapping` (`year`,`month`,`profit_center`,`bu_detail_enterprise_code`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

---

CREATE TABLE `t2_enterprise_master` (
  `bu_detail_enterprise_code` varchar(50) NOT NULL DEFAULT '',
  `bu_detail_enterprise_name` varchar(100) DEFAULT NULL,
  `bu_detail_enterprise_name_eng` varchar(100) DEFAULT NULL,
  `bu_detail_enterprise_name_eng_abb` varchar(50) DEFAULT NULL,
  `bu_detail_order` int(11) DEFAULT NULL,
  `manage_code` varchar(50) DEFAULT NULL,
  `manage_enterprise_name` varchar(100) DEFAULT NULL,
  `manage_enterprise_name_eng` varchar(100) DEFAULT NULL,
  `manage_enterprise_name_eng_abb` varchar(50) DEFAULT NULL,
  `manage_order` int(11) DEFAULT NULL,
  `bpc_code` varchar(50) DEFAULT NULL,
  `latest_rating` varchar(50) DEFAULT NULL,
  `latest_subsidiary_cost_existence` varchar(50) DEFAULT NULL,
  `region_code` varchar(50) DEFAULT NULL,
  `enterprise_start_year_month` date DEFAULT NULL,
  `enterprise_end_year_month` date DEFAULT NULL,
  KEY `idx1_enterprise_master` (`bu_detail_enterprise_code`),
  KEY `idx2_enterprise_master` (`bu_detail_enterprise_code`,`bu_detail_enterprise_name`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8