An insulation resistance detection device detects a voltage at a connection point between a coupling capacitor and a resistance in the case where a frequency signal is outputted to the resistance, and detects an insulation resistance between a ground portion and a power supply path based on a moving average value of a detection voltage. The insulation resistance detection device includes a first determination unit, a resistance detection unit, and a second determination unit. In response to a determination that the pulsation of the detection voltage is attenuated, the resistance detection unit detects the insulation resistance based on, instead of a first moving average value, a second moving average value calculated by using the detection voltage in a second range including a smaller number of detection voltages than in the first range.