A liquid injection instrument includes an internal structural body having a cylindrical body that has a cylindrical shape and has an inner cavity part in which a drug solution is housed, a hollow needle disposed at a distal part of the cylindrical body, a gasket slidable in the cylindrical body, and a plunger that carries out movement operation of the gasket along an axial direction of the cylindrical body. The liquid injection instrument further includes a cylindrical protector disposed on an outer circumferential side of the internal structural body movably along the axial direction of the internal structural body. The protector has an outside engagement piece and the cylindrical body has an inside engagement piece engageable with the outside engagement piece.