To calculate exact overtime hours without evolving a mismatch, or the like between the overtime hours and actual working hours caused by taking days out.

An overtime hours calculation system comprises: a diligence and indolence data input means 5 inputting one-day diligence and indolence data; a one-day actual working hours calculation means 10 calculating one-day actual working hours from the one-day diligence and indolence data; an actual working hours calculation means 11 for a predetermined period calculating actual working hours for the predetermined period by adding the calculated one-day actual working hours; a predetermined working hours calculation means 12 for the predetermined period calculating the predetermined working hours for the predetermined period from predetermined one-day predetermined working hours; and a predetermined hours-exceeded overtime hours calculation means 13 calculating predetermined hours-exceeded overtime hours obtained by referring to a difference between the calculated actual working hours for the predetermined period and the specified working hours for the predetermined period.