Seven-Eleven Japan Co. has failed over the span of several decades to pay at least ¥490 million in overtime to part-time workers at its franchise-operated convenience stores, sources familiar with the matter said Tuesday.

A long-running error in the Seven & I Holdings Co. unit’s calculations for such workers led to the shortfall, according to the sources.

They said the company plans to pay the outstanding overtime to an estimated 30,000 or more employees affected by the error.

Franchise owners shoulder personnel costs for their employees, while wage calculations and actual payments are made on their behalf by Seven-Eleven Japan.

A labor standards inspection office found the calculation error in September, the sources said. The mistake resulted in overtime payments lower than legal standards for certain part-time workers. The lower payments may have started as early as the 1970s, they said.