A liquid medicine administration device includes a prefilled syringe and a gasket biasing member assembly. The biasing member assembly includes: a gasket pressing member configured to press a gasket； a flange attachment member attached to the prefilled syringe； a biasing member housed between the pressing member and the flange attachment member； and an operating member including an engagement holding portion. The pressing member includes an engagement portion to be engaged with the flange attachment member. When the engagement holding portion is located at an abutment position, deformation of a deformable portion is inhibited by abutment between the engagement holding portion and an abutment portion of a deformable portion, and engagement between the engagement portion and the attachment member body portion is held.
