In relay weld diagnostic device, a threshold value determination unit (104) determines a threshold value on the basis of a resistance value that is set for each of a P-pole-side resistor (24) and an N-pole-side resistor (23) provided to charging equipment. A peak value measuring unit (110) measures the peak value of a voltage supplied to a voltage line by an alternating-current signal output circuit (112). A comparison diagnosis unit (111) assesses the state of a relay (14) on the P-pole side and a relay (15) on the N-pole side by comparing the peak value and the threshold value, and makes a diagnosis as to whether the relays (14), (15) are welded on the basis of the assessed state of the relays (14), (15) and the state in which the relays (14), (15) are controlled.