A tape cartridge (C) mounted to a tape printer , comprising: a ribbon feeding core on which an ink ribbon (R) is wound so as to be capable of being fed; and a ribbon winding core that winds the ink ribbon (R) fed from the ribbon feeding core , wherein the ribbon feeding core includes: a core body that winds the ink ribbon (R); and a shaft engaging part formed at an end portion of the core body and engaged with a feeding-side driving shaft of the tape printer , and the shaft engaging part protrudes in an axial direction of the core body from an end surface of the core body and is formed in a crown shape in which a plurality of convex portions and a plurality of concave portions are alternately arranged side by side in a circumferential direction, characterized in that a distal end portion in axial direction of each convex portion is formed in a tapered trapezoidal shape when viewed from the outside in the radial direction, wherein the distal end portion of each convex portion is formed at both corner portions with a chamfer portion serving as a guide slope that guides a spline convex part of the feeding-side driving shaft into the concave portion when the tape cartridge (C) is mounted to the tape printer ; and a flat surface formed between the chamfer portions and serving as a reference surface when the ribbon feeding core is assembled to the tape cartridge (C). a ribbon feeding core on which an ink ribbon (R) is wound so as to be capable of being fed; and a ribbon winding core that winds the ink ribbon (R) fed from the ribbon feeding core , wherein the ribbon feeding core includes: a core body that winds the ink ribbon (R); and a shaft engaging part formed at an end portion of the core body and engaged with a feeding-side driving shaft of the tape printer , and the shaft engaging part protrudes in an axial direction of the core body from an end surface of the core body and is formed in a crown shape in which a plurality of convex portions and a plurality of concave portions are alternately arranged side by side in a circumferential direction, a core body that winds the ink ribbon (R); and a shaft engaging part formed at an end portion of the core body and engaged with a feeding-side driving shaft of the tape printer , and the shaft engaging part protrudes in an axial direction of the core body from an end surface of the core body and is formed in a crown shape in which a plurality of convex portions and a plurality of concave portions are alternately arranged side by side in a circumferential direction, characterized in that a distal end portion in axial direction of each convex portion is formed in a tapered trapezoidal shape when viewed from the outside in the radial direction, wherein the distal end portion of each convex portion is formed at both corner portions with a chamfer portion serving as a guide slope that guides a spline convex part of the feeding-side driving shaft into the concave portion when the tape cartridge (C) is mounted to the tape printer ; and a flat surface formed between the chamfer portions and serving as a reference surface when the ribbon feeding core is assembled to the tape cartridge (C). the distal end portion of each convex portion is formed at both corner portions with a chamfer portion serving as a guide slope that guides a spline convex part of the feeding-side driving shaft into the concave portion when the tape cartridge (C) is mounted to the tape printer ; and a flat surface formed between the chamfer portions and serving as a reference surface when the ribbon feeding core is assembled to the tape cartridge (C). <EOS>