Abstract: A motor controller capable of relaxing the temperature of a heat source which generates heat by driving the motor controller, and capable of realizing miniaturization and low price is provided. In a motor controller in which a power semiconductor module (2) which closely contact a heat sink (1) is mounted on a substrate (4), and a fan (6) is attached to the heat sink (1) by a fan case (8), portions of fins (1g) of the heat sink (1) are formed with a bell mouth (1c) and a circular-arc-shaped guide (1h), and the radiating capability of the fins (1g) is improved by effectively using the wind of the fan (6).