Nikkei newspaper estimated the market share of internet service providers in 2013 in Japan as the below. The total number of broadband connection contracts was 81.34 million. The number of contracts increased by 51.8% from 2012.
OCN of NTT Communications was the largest provider.
I think most of smartphone users use top 3 providers.
|ISP||run by||Market share (%)|
|au one net||KDDI||13.5|
The source of the market share is Nikkei Sangyo Shimbun newspaper, the article on 2014 July 28th.