An apparatus is used for diagnosing the temperature state of a catalyst converter. The catalyst converter includes a catalyst for cleaning an emission, and a conductive carrier for carrying the catalyst. The conductive carrier is energized for temperature rise of the catalyst, and the conductive carrier has a characteristic in which resistance drops with temperature increase. In the apparatus, a first obtaining unit obtains a first parameter having a first correlation with supply power to the conductive carrier for energization of the conductive carrier. A second obtaining unit obtains a second parameter having a second correlation with a temperature of the conductive carrier. A diagnosing unit diagnoses the temperature state of the conductive carrier based on a comparison between the first parameter and the second parameter.