A power conversion device includes a first inverter and a control circuit that controls on/off operations of switches in the first inverter and diagnoses disconnection failures of n-phase windings where n is an integer of three or more. The control circuit generates a control signal to turn on one low-side switch of a first specific phase among n low-side switch in the first inverter, turn off the remaining n−1 low-side switches, and turn off all n high-side switches, supplies the control signal to the n low-side switches and the n high-side switches in a state where a neutral point of a motor is provided, measures n-phase voltages that change depending on patterns of disconnection failures of the n-phase windings, and diagnoses a disconnection failure based on the measured n-phase voltages by referring to a table associating the patterns of the disconnection failures with n-phase voltage levels.