An electric motor-attached speed reducer comprising: a first member; an electric motor arranged to produce rotational motion with respect to the first member; a speed reduction mechanism arranged to transfer the rotational motion obtained from the electric motor while reducing a speed thereof; a second member arranged to rotate relative to the first member at a rotation rate resulting from the speed reduction; a first bearing arranged to support the first and second members (10, 30) such that the first and second members (10, 30) are rotatable relative to each other; and a second bearing arranged to support the first member and a rotor of the electric motor such that the first member and the rotor are rotatable relative to each other, the rotor including a rotor magnet, and a rotor holder arranged to hold the rotor magnet; wherein the speed reduction mechanism includes: a cam; an external gear; a third bearing being flexible; and an internal gear; the external gear and the internal gear are arranged to have different numbers of teeth, mesh with each other, and rotate relative to each other because of the different numbers of teeth; the second bearing is arranged to radially overlap with the rotor holder at a first axial position; and the third bearing is arranged to radially overlap with the rotor holder at a second axial position different from the first axial position, characterized in that the cam is a non-perfect circular cam including an outer circumferential surface being at different distances from a rotation axis of the electric motor at different circumferential positions; the external gear is a flexible external gear arranged to be deformed in accordance with rotation of the non-perfect circular cam; the third bearing is arranged between the non-perfect circular cam and the flexible external gear; the internal gear is arranged in the second member; and at least a portion of the second bearing and at least a portion of the third bearing are arranged axially opposite to each other. a first member; an electric motor arranged to produce rotational motion with respect to the first member; a speed reduction mechanism arranged to transfer the rotational motion obtained from the electric motor while reducing a speed thereof; a second member arranged to rotate relative to the first member at a rotation rate resulting from the speed reduction; a first bearing arranged to support the first and second members (10, 30) such that the first and second members (10, 30) are rotatable relative to each other; and a second bearing arranged to support the first member and a rotor of the electric motor such that the first member and the rotor are rotatable relative to each other, the rotor including a rotor magnet, and a rotor holder arranged to hold the rotor magnet; wherein the speed reduction mechanism includes: a cam; an external gear; a third bearing being flexible; and an internal gear; a cam; an external gear; a third bearing being flexible; and an internal gear; the external gear and the internal gear are arranged to have different numbers of teeth, mesh with each other, and rotate relative to each other because of the different numbers of teeth; the second bearing is arranged to radially overlap with the rotor holder at a first axial position; and the third bearing is arranged to radially overlap with the rotor holder at a second axial position different from the first axial position, characterized in that the cam is a non-perfect circular cam including an outer circumferential surface being at different distances from a rotation axis of the electric motor at different circumferential positions; the external gear is a flexible external gear arranged to be deformed in accordance with rotation of the non-perfect circular cam; the third bearing is arranged between the non-perfect circular cam and the flexible external gear; the internal gear is arranged in the second member; and at least a portion of the second bearing and at least a portion of the third bearing are arranged axially opposite to each other.