A ballpoint pen tip comprising: a writing ball, a holder adapted to hold the writing ball within a tip-end thereof, a tapered portion at a tip-end part of an outer periphery of the holder, a narrowed portion adapted to hold the writing ball by a tip-end of the tapered portion deformed plastically and inwardly, a ball house adapted to be an inner space of the holder at the tapered portion in which the writing ball is inserted, a back hole adapted to be an inner space of the holder extending forward from a rear end of the holder to an ink guide hole, the ink guide hole being adapted to be an inner space of the holder connecting the back hole and the ball house, a plurality of ink grooves respectively formed as grooves penetrating from the ball house to a tip-end part of the back hole, the plurality of ink grooves being distributed at equal intervals in a radial manner at a plurality of positions around the ink guide hole with respect to an axial center of the ink guide hole, an elastic member adapted to bias the writing ball forward being inserted in the back hole, and a biasing portion formed by a rod-shaped tip-end part of the elastic member that extends forwardly, passing through the ink guide hole, so as to contact a rear-end of the writing ball in the ball house and bias it forward; characterized in that: a plurality of inward protrusions, protruding inwardly from the grooves, are formed in the rear-ends of the ink grooves; when the inner diameter of the ink guide hole is A, the inner diameter of the tip-end part of the back hole is B, the diameter of a circle circumscribing the outermost walls of the ink grooves is C, and the diameter of a circle inscribing the inner peripheral surface of the inward protrusions is D, the relationships of A < B < C and D < B are satisfied; and the width in the circumferential direction of each of the ink grooves is larger than the outer diameter of the rod shaped biasing portion. a writing ball, a holder adapted to hold the writing ball within a tip-end thereof, a tapered portion at a tip-end part of an outer periphery of the holder, a narrowed portion adapted to hold the writing ball by a tip-end of the tapered portion deformed plastically and inwardly, a ball house adapted to be an inner space of the holder at the tapered portion in which the writing ball is inserted, a back hole adapted to be an inner space of the holder extending forward from a rear end of the holder to an ink guide hole, the ink guide hole being adapted to be an inner space of the holder connecting the back hole and the ball house, a plurality of ink grooves respectively formed as grooves penetrating from the ball house to a tip-end part of the back hole, the plurality of ink grooves being distributed at equal intervals in a radial manner at a plurality of positions around the ink guide hole with respect to an axial center of the ink guide hole, an elastic member adapted to bias the writing ball forward being inserted in the back hole, and a biasing portion formed by a rod-shaped tip-end part of the elastic member that extends forwardly, passing through the ink guide hole, so as to contact a rear-end of the writing ball in the ball house and bias it forward; characterized in that: a plurality of inward protrusions, protruding inwardly from the grooves, are formed in the rear-ends of the ink grooves; when the inner diameter of the ink guide hole is A, the inner diameter of the tip-end part of the back hole is B, the diameter of a circle circumscribing the outermost walls of the ink grooves is C, and the diameter of a circle inscribing the inner peripheral surface of the inward protrusions is D, the relationships of A < B < C and D < B are satisfied; and the width in the circumferential direction of each of the ink grooves is larger than the outer diameter of the rod shaped biasing portion. a plurality of inward protrusions, protruding inwardly from the grooves, are formed in the rear-ends of the ink grooves; when the inner diameter of the ink guide hole is A, the inner diameter of the tip-end part of the back hole is B, the diameter of a circle circumscribing the outermost walls of the ink grooves is C, and the diameter of a circle inscribing the inner peripheral surface of the inward protrusions is D, the relationships of A < B < C and D < B are satisfied; and the width in the circumferential direction of each of the ink grooves is larger than the outer diameter of the rod shaped biasing portion.