An insulation detecting device includes a control unit configured to monitor a measurement voltage of a flying capacitor charged through a second path, and in a case where a measurement voltage is about 0 V (zero volt), control a switching of switches, control a charging of the flying capacitor through a third path and a measuring of a charged voltage of the flying capacitor charged through the third path, and measure a first measurement voltage in a first time after the switching to the third path and a second measurement voltage in a second time different from the first time, and a calculating unit configured to calculate a ground fault resistance formed between a direct-current power supply and a ground based on the first measurement voltage and the second measurement voltage.