A shaft member coupling structure includes first and second spool members lockably coupled in a condition wherein the spool members coaxially extend with a coupling part in the center, a fitting concave portion axially extends and is formed at the end of the first spool member and a fitting convex portion axially extends and can be fitted into the fitting concave portion and is formed at the end of the second spool member. A coupling pin is inserted into first and second coupling holes. Further, an annular holding groove concave in a circumferential direction is formed in a part in which the coupling hole is formed on the periphery of the first spool member, both ends of the coupling pin inserted into the coupling holes are covered with a ring fitted into the holding groove. Thus, the coupling pin is prevented from falling away.