A motor driving device capable of reliably detecting a malfunction in the performance of a heatsink. The motor driving device includes a heat generating element, a heatsink, an electric power detecting part for detecting a consumed power of the heat generating element, a temperature detecting part for detecting a temperature of the motor driving device, a temperature change calculating part for calculating, as a detected valuation, an amount of change in the temperature within a predetermined time, a reference determination part for determining a reference amount of change in the temperature based on the temperature and the consumed power, and a temperature change judging part for comparing the reference amount of change with the detected amount of change, and judges whether the detected change is different from the reference change.