An inscribed transmission device includes a substantially crescent-shaped space between an internal gear and an external gear circumscribingly in mesh with each other. The device comprises a rolling space holder having two sets of two rollers whose centers are situated on radial axes in the crescent-shaped space on both sides of a straight line connecting centers of the internal and external gears. The internal and external gears are provided with roller rolling portions on which the rollers roll. The two sets of rollers are supported by a block member or side plates. The rolling space holder smoothly moves to keep the crescent-shaped space, thereby preventing unintentional disengagement of the internal and external gears.