A motor with a speed reducer, comprising a motor main body including a rotary shaft; a speed reducer portion including a worm and a worm wheel, a housing including a housing main body and a housing cover, the housing main body accommodating the speed reducer portion and being open to one side in an axial direction of the worm wheel; a circuit board accommodated in the housing, a power component disposed at one side of the circuit board; and a heat-receiving portion disposed at the housing main body, the heat-receiving portion being adjacent to the worm and disposed such that a part of the heat-receiving portion overlaps with the worm as viewed from the side thereof where the opening of the housing main body is disposed, and the heat-receiving portion touching against a face at the other side of the circuit board and receiving heat generated by the power component.