PURPOSE: To provide an extremely simple and high-efficient epoch making gear crank by a method wherein the gear crank comprises right and left gear groups formed such that only the necessary minimum limit number of teeth is formed in a round large gear or an oblong large gear.

CONSTITUTION: For example, only 4T light gear group 9 is formed in the same round large gear 7 as a conventional type and a left gear group 10 having the same number 4T is formed on the opposite side part of a diameter line. As a result, a chain 5 is advanced such that the flange, serving as a pilot rail, of the large gear 7 is nipped in an astride state, in order, in succession by the observe plate 13 and the reverse plate 14 of each block of the chain 5 through engagement with a light gear group 9 and the chain is progressed without the occurrence of deviation and derail of the chain from a flange and the chain is accurately engaged with the next left gear group 10. This is like a monorail, and the chain 5 is prevented from deviation and derail by means of the flange of the large gear 7 serving as a rail.