A charging circuit includes a first power source circuit that supplies charging power to secondary batteries; a switch that controls the supply of the power; and a control device that performs switching control of the switch. The control device detects a voltage difference between the secondary batteries before starting charging; starts a timer and starts serial charging of the secondary batteries; on the condition that the voltage difference before the charging has been equal to or larger than a first threshold, if respective voltage values of the secondary batteries have both become equal to or larger than a second threshold before the timer expires, shortens a remaining time on the timer at that time point; and finishes the serial charging of the secondary batteries at the earlier of a time point when the secondary batteries have both become fully charged, or a time point when the timer has expired.