Yes, he is! (Which explains other of his mangas, Akagi):
reply> He has been playing mahjong since junior high school days, and admitted that though he has rarely lost a game when he was in school, his current level of ability is average. According to him, he has "tournament luck" and has even won mahjong tournaments between mahjong manga artists. He has also participated in professional mahjong matches. He played about two games against Akagi and Kaiji's voice actor Masato Hagiwara, who is known as one of the best mahjong players in the entertainment industry, and made Hagiwara say "I don't think I can beat him."