The present invention relates to an abnormality detection device and an abnormality detection method. The abnormality detection device includes a measuring unit 14c and a determining unit 14d. The measuring unit 14c measures, among a power supply, a capacitor, a load circuit, a switch connecting the power supply to the load circuit, and a ground of a vehicle body which are mounted on a vehicle, a first voltage of the capacitor charged by serially connecting the power supply, the capacitor, and the body ground in a state where the switch is controlled to be turned off. The determining unit 14d determines that the switch is not fixed in an ON state and an insulation resistance of the vehicle is normal when the first voltage measured by the measuring unit 14c is less than a first threshold.