In a shaft coupling structure in which a first shaft and a second shaft are coupled with each other by a combination of a key coupling and a clamp coupling, the first shaft has a cylindrical portion into which the second shaft is inserted and slits which are formed in the cylindrical portion by cutting off portions thereof in the axial direction, and a clamping fastener is mounted on portions of the outer circumference of the first shaft in which the slits are formed and the slit can be also used as a keyway of the key, and the plurality of slits are formed in the circumferential direction.