the internal teeth formed on the inner circumference, a first rotational member with a plurality of circular grooves formed inside that is carried rotatably by the internal tooth member, while keeping a coaxial relation with the internal tooth member, a second rotational member with a plurality of circular grooves formed inside, the plurality of circular grooves being opposed respectively to the plurality of circular grooves for the first rotational member, the second rotational member being carried rotatably by the internal tooth member, while keeping a coaxial relation with the internal tooth member, an external gear having the external teeth formed on the outer circumference, with a plurality of through holes formed inside, the external teeth meshing with the internal teeth of the internal tooth member, the external gearbeing carried between the first rotational member and the second rotational member, a plurality of crankshafts having the first insertion portions to be inserted respectively into the plurality of pairs of opposed circular grooves for the first rotational member and the second rotational member, and the second insertion portions to be inserted respectively into the plurality of through holes for the external gear and orbiting eccentrically the external gear, a plurality of pairs of first bearings for holding the first insertion portions of the crankshafts rotatably with respect to the first rotational member and the second rotational member, respectively, and a plurality of second bearings for holding the second insertion portions of the crankshafts rotatably with respect to the external gear, characterized in that the basic radial load