An actuator comprising: a tubular first housing that is attached to an attached body, a motor provided in the first housing, and including a yoke, a magnet fixed to an inner circumferential surface of the yoke, an armature rotatably provided on an inner side of the yoke in a radial direction, a power supplying part that supplies a current to the armature, and a detecting part that detects a rotation position of the armature; first and second damper members (38, 63), one of which is provided on each of both ends in an axial direction of the motor and absorbs vibration, a drive shaft that is driven and rotated by a rotary shaft of the motor, a speed reduction gear part that is provided between the rotary shaft of the motor and the drive shaft, and that reduces a speed of a rotation of the rotary shaft, a tubular second housing that is provided so as to be capable of being projected/retracted with respect to the first housing, a driven member that is fixed to the second housing and that is moved along an axial direction of the drive shaft in accordance with a rotation of the drive shaft to thereby allows the second housing to be projected/retracted with respect to the first housing, a coil spring that is provided in the first housing and the second housing and that provides a bias force of a direction in which the second housing is protruded with respect to the first housing, wherein the motor is supported in a floating state by the first housing via the first damper member and the second damper member, wherein an internal gear of the speed reduction gear part comprises: a plate part that has a disc shape, a tubular part that has a tubular shape and that extends from an outer circumferential portion of the plate part toward an end of the yoke, and a flange part of which the diameter is increased from the tubular part toward an outer circumference at an end on an opposite side of the plate part, wherein the flange part is butted to another end of the yoke wherein the internal gear is inserted in the yoke at an end thereof. a tubular first housing that is attached to an attached body, a motor provided in the first housing, and including a yoke, a magnet fixed to an inner circumferential surface of the yoke, an armature rotatably provided on an inner side of the yoke in a radial direction, a power supplying part that supplies a current to the armature, and a detecting part that detects a rotation position of the armature; first and second damper members (38, 63), one of which is provided on each of both ends in an axial direction of the motor and absorbs vibration, a drive shaft that is driven and rotated by a rotary shaft of the motor, a speed reduction gear part that is provided between the rotary shaft of the motor and the drive shaft, and that reduces a speed of a rotation of the rotary shaft, a tubular second housing that is provided so as to be capable of being projected/retracted with respect to the first housing, a driven member that is fixed to the second housing and that is moved along an axial direction of the drive shaft in accordance with a rotation of the drive shaft to thereby allows the second housing to be projected/retracted with respect to the first housing, a coil spring that is provided in the first housing and the second housing and that provides a bias force of a direction in which the second housing is protruded with respect to the first housing, wherein the motor is supported in a floating state by the first housing via the first damper member and the second damper member, wherein an internal gear of the speed reduction gear part comprises: a plate part that has a disc shape, a tubular part that has a tubular shape and that extends from an outer circumferential portion of the plate part toward an end of the yoke, and a flange part of which the diameter is increased from the tubular part toward an outer circumference at an end on an opposite side of the plate part, wherein the flange part is butted to another end of the yoke wherein the internal gear is inserted in the yoke at an end thereof. a plate part that has a disc shape, a tubular part that has a tubular shape and that extends from an outer circumferential portion of the plate part toward an end of the yoke, and a flange part of which the diameter is increased from the tubular part toward an outer circumference at an end on an opposite side of the plate part, wherein the flange part is butted to another end of the yoke wherein the internal gear is inserted in the yoke at an end thereof.