A voltage detecting circuit includes a ground potential unit, a DC power supply which is insulated from the ground potential unit, a capacitor to which voltage from the DC power supply is applied, a control unit which controls charging and discharging of the capacitor, a charging circuit that includes the DC power supply, the capacitor and a charging path through which the capacitor is charged, and a discharging circuit that includes the capacitor and a discharging path through which the capacitor is discharged. The control unit includes a calculation unit which calculates a voltage value of the DC power supply, based on an actual capacitance value C of the capacitor which is obtained by both-end voltage of the capacitor corresponding to a first time and the both-end voltage of the capacitor corresponding to a second time.