An electric power steering device includes a motor, a drive pulley that is connected to the output shaft of the motor and includes helical teeth, a driven pulley that is arranged coaxially with a rack shaft and includes helical teeth, and a belt that is wound around these pulleys and includes helical teeth meshing with the respective helical teeth of both pulleys. Internal teeth on a belt each are formed to have a tooth thickness that is reduced toward both ends thereof along the width direction of the belt.