An apparatus for diagnosing a temperature detection unit, mounted in a vehicle having a power conversion circuit including a switching element, a control unit configured to operate the switching element to control a torque of a main rotating machine electrically connected to the power conversion circuit to a demanded torque. The temperature detection unit is configured to detect a temperature of the switching element. In the apparatus, a current supply increasing unit is configured to operate the switching element to increase a current supply to the switching element. A permission unit permits the current supply increasing unit to increase the current supply only when a braking torque is being applied to the vehicle. A diagnostic unit determines that an abnormality is present in the temperature detection unit when it is determined that the detection temperature of the temperature detection unit is out of an acceptable range.