To eliminate any sliding, correctly transmit power, and correspond a large load by preparing plural groups of chains and sprockets setting one group in an engaging condition, and controlling the other group of engagement by a cam mechanism on the basis of the group.

A chain A is wound around a circumference of a circular cone column for about half-around on the basis of a chain guide rail while using the circular cone column, and a chain B is also wound for a half-around in a reverse direction on the base point of the chain guide rail. Such one group (A1, B1) of construction body is prepared. Four chains are moved on a guide rail, and thereby, a radius of the chain is changed continuously. A gear case M having gears C, D engaged with the chains A, B: A1, B1 which are attached to a holder E rotated serving a center of the circular cone column as an input shaft, is arranged on each construction body. A lower part of the circular cone column is formed in a cam shape, and movement of the cam is transmitted to a phase control gear through a control lever.