A liquid medicine ejection tool comprising: a syringe that includes a barrel having a flange at a proximal end portion and a gasket slidable inside the barrel and made of an elastic material; and a gasket pressing tool used by being mounted on the syringe, wherein the gasket pressing tool includes: a gasket pressing member having a gasket-mounting portion, which is capable of entering the lumen portion of the gasket and made of a material harder than the gasket, and a gasket proximal surface pressing portion provided at a proximal end portion of the gasket-mounting portion; a syringe-mounting member that movably accommodates the gasket pressing member and has a proximal barrel-mounting portion that is mountable to a proximal end portion of the barrel having the flange; and a plunger that has a distal end portion entering the syringe-mounting member and is capable of pressing the gasket pressing member in a distal direction, characterized in that the gasket includes a tubular portion, a distal closing portion that closes a distal end of the tubular portion, a lumen portion defined by the tubular portion and the distal closing portion, and an annular rib (73, 74, 75) provided on an outer surface of the tubular portion, the gasket has an original lumen portion axial length S when the gasket is not inserted into the barrel and the gasket pressing member is not mounted, the gasket-mounting portion of the gasket pressing member has an axial length T, and the axial length T is longer than the original lumen portion axial length S, and when the gasket is inserted into the barrel and the gasket pressing member is mounted, a mounting portion distal surface of the gasket-mounting portion of the gasket pressing member abuts on a lumen distal surface of the lumen portion of the gasket, and the gasket proximal surface pressing portion of the gasket pressing member becomes close to or abuts on a proximal surface of the gasket. the gasket pressing tool includes: a gasket pressing member having a gasket-mounting portion, which is capable of entering the lumen portion of the gasket and made of a material harder than the gasket, and a gasket proximal surface pressing portion provided at a proximal end portion of the gasket-mounting portion; a syringe-mounting member that movably accommodates the gasket pressing member and has a proximal barrel-mounting portion that is mountable to a proximal end portion of the barrel having the flange; and a plunger that has a distal end portion entering the syringe-mounting member and is capable of pressing the gasket pressing member in a distal direction, characterized in that the gasket includes a tubular portion, a distal closing portion that closes a distal end of the tubular portion, a lumen portion defined by the tubular portion and the distal closing portion, and an annular rib (73, 74, 75) provided on an outer surface of the tubular portion, the gasket has an original lumen portion axial length S when the gasket is not inserted into the barrel and the gasket pressing member is not mounted, the gasket-mounting portion of the gasket pressing member has an axial length T, and the axial length T is longer than the original lumen portion axial length S, and when the gasket is inserted into the barrel and the gasket pressing member is mounted, a mounting portion distal surface of the gasket-mounting portion of the gasket pressing member abuts on a lumen distal surface of the lumen portion of the gasket, and the gasket proximal surface pressing portion of the gasket pressing member becomes close to or abuts on a proximal surface of the gasket.