Heating value P[W] generated at an IGBT is calculated, and a temperature difference .DELTA.T'.sub.j [.degree. C.] between a temperature T.sub.w [.degree. C.] of cooling water circulating in a cooling system and a temperature T.sub.j [.degree. C.] of an IGBT is calculated based on thermal resistance R [.degree. C./W] of the cooling system. A temperature rise .DELTA.T.sub.j [.degree. C.] with transient influences eliminated is then calculated based on the calculated temperature difference .DELTA.T'.sub.j [.degree. C.], and the temperature T.sub.j [.degree. C.] (=T.sub.w [.degree. C.]+.DELTA.T.sub.j [.degree. C.]) of the IGBT is calculated.