PURPOSE: To improve newness, progressiveness and usefulness by forming a base plate integrally with a crank about a crank shaft, providing half small gears with the same number of teeth at the right and left end parts of the base plate to form separate chain wheels and cranks, and connecting and bonding a back ring to the back face of the separate chain wheels and cranks.

CONSTITUTION: An oblong base plate 5 is integrally formed with a crank 3, and a left half small gear 13 with suitable number of teeth is formed at the left side end part of the base plate 5. Similarly, a light half small gear 14 with the same number of teeth is formed at the right side end part. A back ring 15 is then bonded to the back face of the base plate 5. The right half small gear 14, the left half small gear 13, and a free wheel 10 are connected to each other by a chain 9. In this constitution, tractive force is exerted on the chain 9 depending on the angle of rotation of the right and left cranks 3, 8, adopting a pedal shaft 2 as a lever force point, a crankshaft 4 as a fulcrum, and the parts of the right and left half small gears 13, 14 as operation points.