A planetary gear drive, comprising: an internal gear, a sun gear disposed concentrically within the internal gear, at least one planetary gear in meshed engagement with the internal gear and sun gear, a carrier that rotatably supports the planetary gear, a housing that houses the above gears (2, 3, 4) and carrier, and wherein the carrier has an output-side disc-shaped end plate portion and a cylindrical portion extending to an input side of the drive from the output-side disc-shaped end plate portion, the sun gear is inserted concentrically into the cylindrical portion of the carrier from the input side in a state in which the sun gear is incorporated in a floating state that enables it to move perpendicularly to an axis of the drive; the housing comprises an input-side cylindrical housing and an output-side cylindrical housing; the planetary gear drive comprises an input-side roller bearing and an output-side roller bearing attached to an inner peripheral surface of the housing, said roller bearings (7, 8) being positioned along the axis of the drive, which is a common axis of the internal gear, the carrier and the housing, the output-side roller bearing rotatably supporting the output-side disc-shaped end plate portion on an inner peripheral surface of the output-side cylindrical housing of the housing and the input-side roller bearing supporting the cylindrical portion of the carrier, characterized in that the internal gear is a thin, elastically resilient internal gear that is capable of radially elastic deformation and comprises an annular flange affixed to the housing, a thin, cylindrical portion extending around the inner peripheral surface of the output-side cylindrical housing from the flange to an output side of the drive, and internal teeth formed on an inner peripheral surface of the thin, cylindrical portion, the input-side cylindrical housing and the output-side cylindrical housing are fixed concentrically together with the annular flange of the internal gear clamped therebetween, an annular space is formed between the outer peripheral surface of the thin, cylindrical portion of the internal gear and the inner peripheral surface of the output-side cylindrical housing of the housing to permit outward radial elastic deformation of the thin, cylindrical portion and said roller bearings (7, 8) are tapered roller bearings (7, 8), an input-side portion of the cylindrical portion of the carrier is rotatably supported on the inner peripheral surface of the annular flange of the internal gear and on an inner peripheral surface of the input-side cylindrical housing of the housing by the input-side roller bearing. an internal gear, a sun gear disposed concentrically within the internal gear, at least one planetary gear in meshed engagement with the internal gear and sun gear, a carrier that rotatably supports the planetary gear, a housing that houses the above gears (2, 3, 4) and carrier, and wherein the carrier has an output-side disc-shaped end plate portion and a cylindrical portion extending to an input side of the drive from the output-side disc-shaped end plate portion, the sun gear is inserted concentrically into the cylindrical portion of the carrier from the input side in a state in which the sun gear is incorporated in a floating state that enables it to move perpendicularly to an axis of the drive; the housing comprises an input-side cylindrical housing and an output-side cylindrical housing; the planetary gear drive comprises an input-side roller bearing and an output-side roller bearing attached to an inner peripheral surface of the housing, said roller bearings (7, 8) being positioned along the axis of the drive, which is a common axis of the internal gear, the carrier and the housing, the output-side roller bearing rotatably supporting the output-side disc-shaped end plate portion on an inner peripheral surface of the output-side cylindrical housing of the housing and the input-side roller bearing supporting the cylindrical portion of the carrier, characterized in that the internal gear is a thin, elastically resilient internal gear that is capable of radially elastic deformation and comprises an annular flange affixed to the housing, a thin, cylindrical portion extending around the inner peripheral surface of the output-side cylindrical housing from the flange to an output side of the drive, and internal teeth formed on an inner peripheral surface of the thin, cylindrical portion, the input-side cylindrical housing and the output-side cylindrical housing are fixed concentrically together with the annular flange of the internal gear clamped therebetween, an annular space is formed between the outer peripheral surface of the thin, cylindrical portion of the internal gear and the inner peripheral surface of the output-side cylindrical housing of the housing to permit outward radial elastic deformation of the thin, cylindrical portion and said roller bearings (7, 8) are tapered roller bearings (7, 8), an input-side portion of the cylindrical portion of the carrier is rotatably supported on the inner peripheral surface of the annular flange of the internal gear and on an inner peripheral surface of the input-side cylindrical housing of the housing by the input-side roller bearing.