An insulation resistance detecting apparatus is provided to detect an insulation resistance of a load circuit. The insulation resistance detecting apparatus includes a coupling capacitor of which a first end is connected to the load circuit; a periodic waveform output section which is connected to a second end of the coupling capacitor, and outputs a periodic waveform to the second end; a waveform shaping section which extracts and amplifies a part of the periodic waveform including a peak point; and an arithmetic operation section which determines the insulation resistance of the load circuit on the basis of a first wave height as a wave height value of a waveform outputted from the waveform shaping section and a second wave height value as a wave height value of the periodic waveform.