A method of detecting a fault of a semiconductor device including a power device mounted on a metal base and a drive circuit for driving the power device, the method detecting a fault of the semiconductor device beforehand based on an increase in thermal resistance between the metal base and the power device. A state of the power device is measured immediately before and after the power device is driven by the drive circuit. A temperature difference of the power device before and after driving is calculated according to the result of measurement. An increase in thermal resistance between the metal base and the power device is detected based on the temperature difference and an amount of electricity inputted to the power device in the driving period, and a fault of the semiconductor device is detected beforehand according to the increase.