12 Japanese players were seen on the field towards the end of their World Cup match against South Korea in Bhubaneswar on Tuesday, prompting the International Hockey Federation (FIH) to investigate the matter.
Japan lost the Pool B match 1-2 and the FIH said the result will stand.
"In the last moments of today's FIH Hockey Men's World Cup match between Japan and Korea, the Japanese team had 12 players on the field of play, instead of a maximum of 11 as stipulated in the FIH Rules of Hockey," the FIH said in a statement.
"After the match, the FIH Officials – who have not spotted this situation on the moment – have spoken to the Japanese team who have explained that they hadn't realised this at all and expressed their most sincere apologies."
The FIH officials have also explained the matter to the Korean team.
"FIH is currently investigating the issue to establish how it happened."