A power steering device of the present invention is capable of detecting a contact weld of phase open relays with a simple diagnostic program without use of the circuit exclusive for the contact weld detection purpose. The power steering device has: a current detection circuit (55) configured to detect currents flowing in an inverter (54); and phase open relays (58v, 58w) switchable at least any two of the three phases of a motor (M) between a conduction state and an interruption state. In addition, the power steering device has an abnormality monitor section (57) configured to output vector control command signals (Vd*, Vq*) to supply only a d-axis current in a positive side direction toward the motor, with all of phase open relays (58v, 58w) in an interruption state, and to determine an abnormality in the phase open relays (58v, 58w) in a case where a current value is detected by a current detection circuit (55).