To drastically reduce the slipping between a belt and a pulley in a variable diameter pulley, reduce a load at the time of start, increase an amount of change, increase a gear ratio and a reduction ratio, employ a material which causes no slippage, remove the uneven tension and distortion, and eliminate a deviation between the optimal tension and a base line.

The engagement of the variable diameter pulley is increased and in place of the teeth of a gear, balls are engaged with each other in a disk shape to change a speed. The diameter of the pulley is changed at the time of start depending on a load. A disk is placed between pulleys for move in place of a belt. The belt is adjusted in the core of a V-belt wheel.