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 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 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 . <EOS>