A shaft coupling structure, comprising: a first shaft (45a; 46; 80) having a first coupling portion (54; 65; 81); a second shaft (13a; 30; 45b) having a second coupling portion (24b; 44; 65a) having a cylindrical shape into which the first coupling portion is inserted; and a weld bead portion (52a; 52b; 61) that welds and fixes together an end portion in an axial direction of the second coupling portion (24b; 44; 65a) and a portion of an outer peripheral surface of the first shaft (45a; 46; 80) that is exposed from the second coupling portion (24b; 44; 65a); the first coupling portion (54; 65; 81), on an outer peripheral surface thereof, having an outer peripheral side concave-convex portion (47a; 58; 82) having a concave-convex shape in a circumferential direction and is arranged on an end portion on a front side in an insertion direction of the first coupling portion (54; 65; 81), and an annular concave groove (57; 57a; 57b) that is arranged farther on a rear side in the insertion direction of the first coupling portion (54; 65; 81) than the outer peripheral side concave-convex portion (47a; 58; 82); and the second coupling portion (24b; 44; 65a), on an inner peripheral surface thereof, having an inner peripheral side concave-convex portion (27; 41; 83) having an concave-convex shape in a circumferential direction and constitutes a torque transmitting portion (69; 69a; 69b) by engaging with the outer peripheral side concave-convex portion (47a; 58; 82) with a concave-convex engagement, and a convex embossed portion (62; 62a; 62b) that is arranged on an inner side of the annular concave groove (57; 57a; 57b) and constitutes a retaining portion (70; 70a; 70b), and, in a portion of an outer peripheral surface thereof that coincide with the embossed portion (62; 62a; 62b), having a concave portion; the weld bead portion (52a; 52b; 61), the retaining portion (70; 70a; 70b), and the torque transmitting portion (69; 69a; 69b) being arranged side-by-side in this order in an axial direction of the first shaft (45a; 46; 80); characterized in that gaps in the axial direction of the first shaft (45a; 46; 80) are provided between inner side surfaces of the annular concave groove (57; 57a; 57b) that face each other and the embossed portion (62; 62a; 62b). a first shaft (45a; 46; 80) having a first coupling portion (54; 65; 81); a second shaft (13a; 30; 45b) having a second coupling portion (24b; 44; 65a) having a cylindrical shape into which the first coupling portion is inserted; and a weld bead portion (52a; 52b; 61) that welds and fixes together an end portion in an axial direction of the second coupling portion (24b; 44; 65a) and a portion of an outer peripheral surface of the first shaft (45a; 46; 80) that is exposed from the second coupling portion (24b; 44; 65a); the first coupling portion (54; 65; 81), on an outer peripheral surface thereof, having an outer peripheral side concave-convex portion (47a; 58; 82) having a concave-convex shape in a circumferential direction and is arranged on an end portion on a front side in an insertion direction of the first coupling portion (54; 65; 81), and an annular concave groove (57; 57a; 57b) that is arranged farther on a rear side in the insertion direction of the first coupling portion (54; 65; 81) than the outer peripheral side concave-convex portion (47a; 58; 82); and the second coupling portion (24b; 44; 65a), on an inner peripheral surface thereof, having an inner peripheral side concave-convex portion (27; 41; 83) having an concave-convex shape in a circumferential direction and constitutes a torque transmitting portion (69; 69a; 69b) by engaging with the outer peripheral side concave-convex portion (47a; 58; 82) with a concave-convex engagement, and a convex embossed portion (62; 62a; 62b) that is arranged on an inner side of the annular concave groove (57; 57a; 57b) and constitutes a retaining portion (70; 70a; 70b), and, in a portion of an outer peripheral surface thereof that coincide with the embossed portion (62; 62a; 62b), having a concave portion; the weld bead portion (52a; 52b; 61), the retaining portion (70; 70a; 70b), and the torque transmitting portion (69; 69a; 69b) being arranged side-by-side in this order in an axial direction of the first shaft (45a; 46; 80); characterized in that gaps in the axial direction of the first shaft (45a; 46; 80) are provided between inner side surfaces of the annular concave groove (57; 57a; 57b) that face each other and the embossed portion (62; 62a; 62b).