A determining device determining electrical leakage in a specified circuit having a voltage source, includes: an AC-signal-output unit that outputs an AC signal to a signal line connected to the specified circuit; a detecting unit that detects a peak value of the AC signal; and a determining unit determining electrical leakage of the specified circuit based on the peak value. The specified circuit includes switching elements switching the connection between the voltage source and a specified voltage to be conduction or blocking; and a determining device prohibits overlapping of the point of time when the switching elements are switched from conduction to blocking or the point of time when switched from the blocking state to the conducting state, and the period from the point of time when the AC-signal-output unit starts outputting the AC signal until the detecting unit detects the peak value of the AC signal.