An eccentric rocking type reduction gear comprising: an internal gear; two external gears (5, 6) meshed with the internal gear; a crank shaft supporting both of the external gears (5, 6); and a carrier (4, 8) rotating together with a rotation of both of the external gears (5, 6), the crank shaft comprising first and second cylindrical cams (31, 32) disposed around an axial line of the crank shaft with respective phases being shifted by 180 degrees and in a manner offset from a rotation center of the crank shaft, both of the external gears (5, 6) being supported by the first and second cylindrical cams (31, 32), respectively, in a freely rotatable manner and in a manner revolvable around an axial line of the internal gear, the carrier (4, 8) comprising a plurality of output pins fastened around an axial line of the carrier (4, 8) at an equal interval, each of the output pins being engaged with a plurality of through-holes (51, 61) provided in both of the external gears (5, 6) to be linked with a rotation movement of both of the external gears (5, 6), the crank shaft being rotated as an input shaft and either one of the internal gear and the carrier (4, 8) being rotated as an output shaft, characterized by first and second eccentric holes (36, 37) which run in an axial direction of the crank shaft and which are in communication with each other being formed in the crank shaft, the first eccentric hole running from a first end face of the crank shaft to a center position of the crank shaft in the axial direction, and being disposed in a manner offset in a same direction as that of the first cylindrical cam, and the second eccentric hole running from a second end face of the crank shaft to the center position of the crank shaft in the axial direction, and being disposed in a manner offset in a same direction as that of the second cylindrical cam. an internal gear; two external gears (5, 6) meshed with the internal gear; a crank shaft supporting both of the external gears (5, 6); and a carrier (4, 8) rotating together with a rotation of both of the external gears (5, 6), the crank shaft comprising first and second cylindrical cams (31, 32) disposed around an axial line of the crank shaft with respective phases being shifted by 180 degrees and in a manner offset from a rotation center of the crank shaft, both of the external gears (5, 6) being supported by the first and second cylindrical cams (31, 32), respectively, in a freely rotatable manner and in a manner revolvable around an axial line of the internal gear, the carrier (4, 8) comprising a plurality of output pins fastened around an axial line of the carrier (4, 8) at an equal interval, each of the output pins being engaged with a plurality of through-holes (51, 61) provided in both of the external gears (5, 6) to be linked with a rotation movement of both of the external gears (5, 6), the crank shaft being rotated as an input shaft and either one of the internal gear and the carrier (4, 8) being rotated as an output shaft, characterized by first and second eccentric holes (36, 37) which run in an axial direction of the crank shaft and which are in communication with each other being formed in the crank shaft, the first eccentric hole running from a first end face of the crank shaft to a center position of the crank shaft in the axial direction, and being disposed in a manner offset in a same direction as that of the first cylindrical cam, and the second eccentric hole running from a second end face of the crank shaft to the center position of the crank shaft in the axial direction, and being disposed in a manner offset in a same direction as that of the second cylindrical cam.