The man conducted medical interviews, examined electrocardiograms and explained check-up results to people in 2010-2011 at Takashimadaira Chuo General Hospital in Tokyo, weekend media reports said.
The man was dispatched to the hospital through an employment agency and is suspected of being involved in the treatment of 2,363 people, the mass-circulation Yomiuri Shimbun and other media reported.
Broadcaster TBS said allegations he is not qualified came to light after a medical exam study school where he taught contacted the hospital.
The hospital has apologised for the incident and asked all those seen by the man, who is reportedly in his forties and worked part time, to go back for re-testing.
“We will thoroughly check the original copies of medical licences of all doctors including those working on a part-time basis,” it said in a statement posted on its website.