To provide a controller cooling structure of an electric power steering device that can effectively cool a controller by raising radiation efficiency of a heat sink without causing upsizing or heavy weight.

A cooling structure of a controller 10 of an electric power steering device that includes the controller 10 that detects steering force input from a steering wheel to a steering mechanism through a steering shaft, and makes an electric motor generate necessary auxiliary steering force by driving control of the electric motor according to a detected value. The controller 10 is contacted with a heatsink 14 and a thin wall part 14a of concave shape is formed on a counter contact surface of the heatsink 14.