An electronic control unit includes: inverter circuits of first and second systems driving a multiphase motor with coil sets on a coil set basis; first and second phase potential detecting circuits detecting potentials of at least one phase of current supply paths from the inverter circuits to the coil set; and a diagnosis apparatus detecting a failure based on potentials detected by the first and second phase potential detecting circuits. Under condition that the inverter circuit of the first system turns ON the upper or lower arm switching element corresponding to one phase of the first coil set and turns OFF the other one so that the inverter circuit of the second system has a high-impedance output, when the potential detected by the second phase potential detecting circuit corresponds to an output potential of the inverter circuit of the first system, the diagnosis apparatus determines that a short-circuit failure occurs.