A reduction gear (W1,W2,..,W6) that is configured to reduce a rotation speed input to an input shaft provided with an eccentric portion, and output the reduced rotation speed from an output shaft, the reduction gear comprising: an inner trochoid that has a substantially tubular shape penetrating at both ends thereof and has a wavy curved surface formed on an inner side circumferential surface thereof; an inner trochoid holder that is designed to hold the inner trochoid; an outer trochoid that is disposed inside the tubular inner trochoid, has a substantially tubular shape penetrated at both ends thereof, and has a wavy curved surface formed on an outer side circumferential surface thereof; a rotation restriction unit (4, 5, 11b) that is designed to restrict rotation of the outer trochoid around its axis; and an output pin that is pinched between the outer side circumferential surface of the outer trochoid and the inner side circumferential surface of the inner trochoid, characterized in that the outer trochoid is connected to the eccentric portion of the input shaft and, when the input shaft rotates, the outer trochoid receives eccentric input from the input shaft and at the same time the rotation around the axis thereof is restricted by the rotation restriction unit (4, 5, 11b), which makes the outer trochoid only revolve around an axis of rotation of the input shaft, when the input shaft rotates, the inner trochoid holder transmits eccentric input generated by the rotation of the input shaft to the inner trochoid to make the inner trochoid revolve around the axis of rotation of the input shaft and at the same time to make the inner trochoid rotate around an axis thereof in the same direction as a direction of the rotation of the input shaft, and the output pin has one end secured to the output shaft and, in response to the outer trochoid's revolution around the axis of rotation of the input shaft and the inner trochoid's revolution around the axis of rotation of the input shaft and rotation around the axis thereof, the output pin is made to revolve around the axis of the input shaft in the same direction as the direction of rotation of the input shaft to thereby rotate the output shaft in the same direction as the direction of rotation of the input shaft. an inner trochoid that has a substantially tubular shape penetrating at both ends thereof and has a wavy curved surface formed on an inner side circumferential surface thereof; an inner trochoid holder that is designed to hold the inner trochoid; an outer trochoid that is disposed inside the tubular inner trochoid, has a substantially tubular shape penetrated at both ends thereof, and has a wavy curved surface formed on an outer side circumferential surface thereof; a rotation restriction unit (4, 5, 11b) that is designed to restrict rotation of the outer trochoid around its axis; and an output pin that is pinched between the outer side circumferential surface of the outer trochoid and the inner side circumferential surface of the inner trochoid, characterized in that the outer trochoid is connected to the eccentric portion of the input shaft and, when the input shaft rotates, the outer trochoid receives eccentric input from the input shaft and at the same time the rotation around the axis thereof is restricted by the rotation restriction unit (4, 5, 11b), which makes the outer trochoid only revolve around an axis of rotation of the input shaft, when the input shaft rotates, the inner trochoid holder transmits eccentric input generated by the rotation of the input shaft to the inner trochoid to make the inner trochoid revolve around the axis of rotation of the input shaft and at the same time to make the inner trochoid rotate around an axis thereof in the same direction as a direction of the rotation of the input shaft, and the output pin has one end secured to the output shaft and, in response to the outer trochoid's revolution around the axis of rotation of the input shaft and the inner trochoid's revolution around the axis of rotation of the input shaft and rotation around the axis thereof, the output pin is made to revolve around the axis of the input shaft in the same direction as the direction of rotation of the input shaft to thereby rotate the output shaft in the same direction as the direction of rotation of the input shaft.