PURPOSE: To provide a compact and economical shaft connecting mechanism capable of attaching easily removably a rotary shaft without any play during running.

CONSTITUTION: A roll shaft 21 having on the outer peripheral surface, a pair of flat surfaces 22A and a pair of arcuate surfaces 22B is fitted in a fit hole 37 having first opposing surfaces 38A opposed through a gap to the respective flat surfaces 22A and second opposing surfaces 38B opposed through a gap to the respective arcuate surfaces 22B to transmit torque. First and second plungers 40, 50 communicating to each other through a communicating path 60 are provided. As the first plunger 40 pressed by the flat surface 22A retreats, the second plunger 50 advances to press the arcuate surface 22B. Thus, the roll shaft 21 is connected to the fit hole 37. When phases fit each other in non-driving time, the second plunger 50 retreats by a compression coil spring 70 and the first plunger 40 advances.