A temperature rise of a semiconductor switching element, which is part of a power conversion device such as an inverter, is estimated by an extremely simple method to assess the degradation and remaining lifetime of the semiconductor switching element.In a heat generation amount calculation unit 12 in a calculation processor 3, current command values Id* and Iq* and voltage command values vu*, vv* and vw* are used to calculate a chip loss. First, current values iu*, iv* and iw* of all output phases are estimated from the current command values. The ON/OFF loss of the chip is represented by a function of an estimated value for a current flowing in each output phase, and the loss can be derived by integration with a PWM carrier frequency f. In addition, with respect to a conduction loss, it is necessary to integrate a conduction time with the estimated current value and a saturation voltage, which is a function of the estimated current value. In this case, the conduction time is calculated from a relationship between a carrier amplitude and the voltage command value in each control cycle fsw of the switching element. In addition, an ambient temperature sensor is added to calculate an absolute temperature.