Nikkei press reported 2015 cement market share in Japan. According to it, the top supplier was Taiheiyo Cement (47%).
Figure: 2015 Cement Market Share in Japan
|Manufacturer||Market share (%)|
|Sumitomo Osaka Cement||19.6|
Source is Nikkei Sangyo Shimbun newspaper, the article on 2016 July 25th.