PURPOSE: To obtain a cooling unit for motor system including a constitution for detecting the cooling capacity wherein conduits of a motor and a controller are coupled in series and a temperature sensor is arranged in the conduit of controller.

CONSTITUTION: In the cooling unit, a cooling medium flows from a conduit inlet A1 of a motor into a conduit A and passes through conduits B, C, coupling between a conduit outlet A2 of the motor and a conduit inlet B1 of a controller to a conduit outlet B2 thus absorbing heat from the motor and controller. Furthermore, a temperature sensor D2 is disposed at the conduit outlet B2 of controller in order to detect cooling state of controller with good timing. Motor and controller are cooled simultaneously and cooling state is detected for both motor and controller only by means of a temperature sensor on the controller side and at the time of abnormal heating, both motor and controller are disabled simultaneously. This constitution allows constant detection of cooling state for both motor and controller.