To provide a rotary shaft coupling mechanism which is high in strength as compared with a conventional one, on which a female screw for fastening can be formed without lengthening the entire length of a coupling and with which the strengthening and the downsizing of the mechanism are attained.

In the rotary shaft coupling mechanism provided with a coupling 1 on which an oval hole is formed by forming a pair of liner grooves 12 by flattening by respectively removing a portion of an inner peripheral surface in positions opposing to each other of the inner peripheral surface of a cylindrical hole 11 and inserting a liner into each liner groove 12 and fixing it, the forming region of a removing part 13 is minimized by forming the removing part 13 for machining the liner groove 12 only in the neighborhood of both end parts in the width direction of the liner groove 12 and the female screw 15 can be formed without lengthening the entire length of the coupling 1 by forming the female screw 15 in a position excepting the portion in which the removing part 13 is formed.