Police on Tuesday arrested a former employee of a Tokyo medical recruitment agency on suspicion of illegally copying the personal data of around 17,000 doctors and nurses.
Kengo Mikami, 36, is suspected of copying the confidential information on or around May 30, 2012, while working as a systems engineer for the company now called MRT Inc., investigative sources said.
The data, copied from an online storage service, included names, addresses, dates of birth, academic records and details of workplaces.
Mikami has denied wrongdoing.
He is also believed to have been involved in a project to found a new recruitment agency, after quitting the Tokyo firm in the summer of 2012.