A tape cartridge mounted to a tape printer, comprising: a ribbon feeding core on which an ink ribbon is wound so as to be capable of being fed; and a ribbon winding core that winds the ink ribbon fed from the ribbon feeding core, wherein the ribbon feeding core includes: a core body that winds the ink ribbon; 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 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. a ribbon feeding core on which an ink ribbon is wound so as to be capable of being fed; and a ribbon winding core that winds the ink ribbon fed from the ribbon feeding core, wherein the ribbon feeding core includes: a core body that winds the ink ribbon; 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; 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 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. 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 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.