If a main switch and a sub-switch are OFF and ON respectively, a comparator compares a source voltage value of the main switch to an output voltage value of a direct-current power source. In the same case, the comparator compares an end-to-end voltage value between a source and a drain of the main switch to a threshold value. A voltage value of the voltage applied to one end of the sub-switch is lower than the voltage value at the drain of the main switch. The output voltage value of the direct-current power source is lower than the voltage value. The threshold value is lower than the difference between the voltage value at the drain of the main switch and the voltage.