A controller of a power converter has: an average loss calculator (202) calculating an average loss in a semiconductor device; and a partial temperature variation estimation part (204), while regarding the semiconductor device as a thermal network including at least one combination of a thermal resistance and a thermal time constant, estimating a partial temperature variation of the combination from a loss in the semiconductor device and the combination of the thermal resistance and the thermal time constant. The partial temperature variation estimation part (204) estimates an average temperature from the loss, the thermal resistance, and the thermal time constant; extracts a pulsation envelope temperature exceeding the maximum value of a pulsation temperature dependent on the average loss and the pulsation frequency; and estimates a temperature variation in the semiconductor device by adding the average temperature and the pulsation envelope temperature.