PURPOSE: To facilitate a climbing running by connecting a coupling neck shaft of a curved bent crank with an interlock shaft of a large gear through the intermediary of a connecting rod.

CONSTITUTION: During a rotating cycle in which a pedal 12 of a bicycle is depressed, a zone in which the pedal 12 can be easily depressed and an effective and appropriate strong drive force is generated, is such a sector zone that curved bent cranks A, B vertically move over an angle range of 90deg. from an upward inclined angle of about 45deg. to a downward inclined angle of about 45deg. When the cranks A, B are at an upward inclined angle of about 45deg., if the pedal 2 and a pedal shaft 5 are depressed by a large force, a coupling shaft C as an acting point strongly tows a connecting rod D connected thereto with a very high traction force about a crank shaft 6 as a fulcrum. Subsequently, as the curved bent crank shafts A, B and the connecting rod D cross with each other, and are inclined, the internal and external angles are changed and shifted so as to the traction force is gradually increased.