This speed reducer with an electric motor has: a hollow shaft that extends in an axial direction around a first center axis; a cylindrical casing fixed to the hollow shaft; a fixing part that is relatively stationary with respect to the casing; an electric motor that generates a rotation motion with respect to a second center axis parallel to the first center axis, on the radial outside of the hollow shaft; a speed reduction mechanism that decelerates and transmits the rotation motion by the electric motor; an output part that rotates with respect to the first center axis at a rotation speed after the deceleration; and a torque sensor that is connected to the casing and the fixing part. The torque sensor has: a strain body that has an annular outer ring and an annular inner ring and is elastically deformable; and a plurality of strain sensors. The outer ring is located at an end portion on the radial outside of the torque sensor, and is connected to one of the casing and the fixing part. The inner ring is located at an end portion on the radial inside of the torque sensor, and is connected to the other of the casing and the fixing part. Each of the plurality of strain sensors is at least partially located in a radial direction between the outer ring and the inner ring.