Tax authorities have found that a central suspect in an election scandal involving a major hospital chain failed to report about ¥100 million in taxable income in 2006, sources said Tuesday.

Torao Tokuda, the 76-year-old founder of the Tokushukai group who is now battling amyotrophic lateral sclerosis, an incurable disease, was found to have hidden rebates from general contractors worth that amount in connection with the group's hospital construction, the sources said.

Tokuda, a former Lower House lawmaker, is suspected to have played a leading role in the election scandal, which involved illegal provision of cash and other benefits to Tokushukai workers who campaigned for his son, Takeshi, for the December 2012 Lower House election. By law, all election campaign work must be voluntary.