A liquid medicine administration device 1 includes a prefilled syringe 10 and a gasket biasing member assembly 20. The biasing member assembly includes: a gasket pressing member 5 that presses a gasket 3; a flange attachment member 7 attached to the prefilled syringe; a biasing member 6 housed between the pressing member and the flange attachment member; and an operating member 9 including an engagement holding portion 93. The pressing member 5 includes an engagement portion 541 to be engaged with the flange attachment member, the engagement portion 541 includes an abutment portion 542, and deformation is regulated by receiving an engagement state holding portion, and an engagement state between the engagement portion 541 and the flange attachment member 7 is held.