A torque transmission shaft, comprising: a shaft (23c-e) having a hollow shape and comprising a yoke portion provided on one end portion in an axial direction; a slit extending in the axial direction and provided on the other end portion in the axial direction; and a female serration (32a-b) provided on an inner peripheral surface of the other end portion in the axial direction; and a clamp having a partial cylindrical shape and comprising a non-continuous portion arranged at one location in a circumferential direction; and a pair of flange portions arranged on both sides of the non-continuous portion in the circumferential direction and respectively having an installation hole into which a tightening member is inserted; and the clamp being externally fitted around the other end portion in the axial direction of the shaft (23c-e), and capable of reducing a diameter of the other end portion in the axial direction of the shaft (23c-e) by reducing a width dimension of the non-continuous portion, the shaft (23c-e) has a small-diameter cylindrical portion (31a-b) at the other end portion in the axial direction, the one end portion in the axial direction of the slit is located farther on the one side in the axial direction than the small-diameter cylindrical portion (31a-b), and the one end portion in the axial direction of the slit is positioned in a portion having an outer diameter dimension that is larger than that of the small-diameter cylindrical portion (3 la-b), characterized in that the female serration (32a-b) is provided only on an inner peripheral surface of the small-diameter cylindrical portion (3 la-b), and the one end portion in the axial direction of the slit is positioned farther on one side in the axial direction than one end edge in the axial direction of the female serration (32a-b) and in a portion (29b, 52) having an outer diameter dimension and inner diameter dimension that are larger than those of the small-diameter cylindrical portion (31a-b). a shaft (23c-e) having a hollow shape and comprising a yoke portion provided on one end portion in an axial direction; a slit extending in the axial direction and provided on the other end portion in the axial direction; and a female serration (32a-b) provided on an inner peripheral surface of the other end portion in the axial direction; and a clamp having a partial cylindrical shape and comprising a non-continuous portion arranged at one location in a circumferential direction; and a pair of flange portions arranged on both sides of the non-continuous portion in the circumferential direction and respectively having an installation hole into which a tightening member is inserted; and the clamp being externally fitted around the other end portion in the axial direction of the shaft (23c-e), and capable of reducing a diameter of the other end portion in the axial direction of the shaft (23c-e) by reducing a width dimension of the non-continuous portion, the shaft (23c-e) has a small-diameter cylindrical portion (31a-b) at the other end portion in the axial direction, the one end portion in the axial direction of the slit is located farther on the one side in the axial direction than the small-diameter cylindrical portion (31a-b), and the one end portion in the axial direction of the slit is positioned in a portion having an outer diameter dimension that is larger than that of the small-diameter cylindrical portion (3 la-b), characterized in that the female serration (32a-b) is provided only on an inner peripheral surface of the small-diameter cylindrical portion (3 la-b), and the one end portion in the axial direction of the slit is positioned farther on one side in the axial direction than one end edge in the axial direction of the female serration (32a-b) and in a portion (29b, 52) having an outer diameter dimension and inner diameter dimension that are larger than those of the small-diameter cylindrical portion (31a-b).