This study explores the actual conditions of unpaid overtime and its determinants by using four panel data sets. The analysis clarified the following four findings. First, the average unpaid overtime working hours for four panel data points fall within the range of the difference of the working hours between the Labour Force Survey and the Monthly Labour Survey and the difference in the working hours between the Labour Force Survey and the Basic Survey on Wage Structure. Second, in an economic recession, while unpaid overtime increases, paid overtime decreases. Third, sudden incoming work, high norms and goals, heavy responsibilities and authority, and environments where it is difficult to leave, all increase unpaid overtime. Lastly, in the case of male workers, lost wages from unpaid overtime in one month range from 27 thousand yen to 66 thousand yen. In the case of female workers, the range is about 15 thousand yen to 42 thousand yen.