Nikkei Press estimated the market share of ad agencies in Japan in 2015. According to it, Dentsu has the biggest share (25.3%). The industry-wide sales in 2015 were 6 million yen.
Figure: 2015 Ad Agency Market Share in Japan
|Agency||Market share (%)|
|East Japan Marketing & Communications||1.7|
Source is Nikkei Sangyo Shimbun newspaper, the article on 2016 July 25th.