A reduction gear comprising: a first bearing (16,16B,16C,16D,16E,16F,16G) that is mounted on a supporting member (10, 10A); an output member that is supported by the first bearing (16,16B,16C,16D,16E,16F,16G) and rotates about a first rotation axis (Al); an internal gear that is coupled to the output member and rotates about the first rotation axis together with the output member; a second bearing that is mounted on the supporting member (10,10A) on an outer side relative to the first bearing (16,16B,16C,16D,16E,16F,16G) in a radial direction; and a pinion gear that is supported by the second bearing, rotates about a second rotation axis different from the first rotation axis (Al), and is engaged with the internal gear, wherein at least a part of the second bearing is located on a side closer to the internal gear with respect to a plane passing through an end portion of the first bearing (16,16B,16C,16D,16E,16F,16G) that is more distant from the internal gear in an axial direction parallel to the second rotation axis and being orthogonal to the second rotation axis, characterized in that the pinion gear includes a tubular outer shaft and an inner shaft at least a part of which is located on an inner side of the outer shaft and that rotates together with the outer shaft, the outer shaft has a small-diameter portion that is in contact with an inner circumferential surface of an inner ring of the second bearing and a large-diameter portion that is engaged with the internal gear, the maximum outer diameter of the large-diameter portion is larger than the bore diameter of the inner ring, an end surface of large-diameter portion is in contact with the second bearing, the inner shaft has a base portion located on an inner side of the small-diameter portion and a torque transmitting portion located on an inner side of the large-diameter portion, the base portion has a first fitting surface that is a cylindrical outer circumferential surface, the small-diameter portion has a second fitting surface that is a cylindrical inner circumferential surface and is in contact with the first fitting surface, one of an outer circumferential surface of the torque transmitting portion and an inner circumferential surface of the large-diameter portion has a groove that extends in the axial direction, and the other of the outer circumferential surface of the torque transmitting portion and the inner circumferential surface of the large-diameter portion has a projection that extends in the axial direction and fits into the groove. a first bearing (16,16B,16C,16D,16E,16F,16G) that is mounted on a supporting member (10, 10A); an output member that is supported by the first bearing (16,16B,16C,16D,16E,16F,16G) and rotates about a first rotation axis (Al); an internal gear that is coupled to the output member and rotates about the first rotation axis together with the output member; a second bearing that is mounted on the supporting member (10,10A) on an outer side relative to the first bearing (16,16B,16C,16D,16E,16F,16G) in a radial direction; and a pinion gear that is supported by the second bearing, rotates about a second rotation axis different from the first rotation axis (Al), and is engaged with the internal gear, wherein at least a part of the second bearing is located on a side closer to the internal gear with respect to a plane passing through an end portion of the first bearing (16,16B,16C,16D,16E,16F,16G) that is more distant from the internal gear in an axial direction parallel to the second rotation axis and being orthogonal to the second rotation axis, the pinion gear includes a tubular outer shaft and an inner shaft at least a part of which is located on an inner side of the outer shaft and that rotates together with the outer shaft, the outer shaft has a small-diameter portion that is in contact with an inner circumferential surface of an inner ring of the second bearing and a large-diameter portion that is engaged with the internal gear, the maximum outer diameter of the large-diameter portion is larger than the bore diameter of the inner ring, an end surface of large-diameter portion is in contact with the second bearing, the inner shaft has a base portion located on an inner side of the small-diameter portion and a torque transmitting portion located on an inner side of the large-diameter portion, the base portion has a first fitting surface that is a cylindrical outer circumferential surface, the small-diameter portion has a second fitting surface that is a cylindrical inner circumferential surface and is in contact with the first fitting surface, one of an outer circumferential surface of the torque transmitting portion and an inner circumferential surface of the large-diameter portion has a groove that extends in the axial direction, and the other of the outer circumferential surface of the torque transmitting portion and the inner circumferential surface of the large-diameter portion has a projection that extends in the axial direction and fits into the groove.