PURPOSE: To provide a double mechanism having a compact constitution and in which highly rotational, high torque, and highly efficient power transmission is possible, and also an axis direction dimension can be lessened.

CONSTITUTION: A mechanism is composed of a first internal gear 21 rotatable on a main shaft C1, an external gear 22 rotatably mounted on an auxiliary shaft C2 eccentric to the main shaft C1 and engaging the first internal gear 21, and a second internal gear 23 rotatably mounted on a sub-shaft C3 eccentric to the auxiliary shaft C2 and engaging the external gear 22. The numbers of tooth of the first and second internal gears 21 and 23 are the same, and the auxiliary shaft C2 and the sub-shaft C3 are possible to revolve as one body around the main shaft C1.