In an insulation resistance detection apparatus, a voltage determiner determines whether a detection voltage, which has changed by at least a preset value, becomes stable at around a changed voltage value. A resistance determiner determines a value of an insulation resistance between a ground portion and a power-supply path based on a currently sampled value of the detection voltage instead of a moving average in response to determination that the detection voltage, which has changed by at least the preset value, becomes stable at around the changed voltage value.