A liquid container manufacturing method includes: a liquid blow molding step of molding a preform (2) into a liquid container (C); and a headspace formation step of forming a headspace (H), by discharging, in a state in which a liquid supply path is closed, a liquid (L) out of the liquid container (C) through a discharge port (28a), the discharge accomplished by drawing-in by a liquid drawing source (33). In the headspace formation step, a pressurized gas is supplied from a pressurized gas supply source (38) into the liquid container (C) through a gas supply port (23c) to thereby assist the discharge, by the drawing-in, of the liquid (L) out of the liquid container (C) through the discharge port (28a), and, when a pressure in a discharge path has increased to a set value, the supply of the pressurized gas by the pressurized gas supply source (38) is stopped.