Nikkei press reported 2015 gasoline market share in Japan. According to it, the top supplier is JX Energy (13%).
Figure: 2015 Gasoline Market Share in Japan
|Manufacturer||Market share (%)|
|JX Energy "ENEOS"||33.5|
|Tonen General "Esso" "Mobil"||20.6|
Source is Nikkei Sangyo Shimbun newspaper, the article on 2016 July 25th.