A method of manufacturing a liquid container, the liquid container including a liquid containing chamber in which a liquid can be contained, an air communicating passage allowing the liquid containing chamber to communicate with air, a liquid supply port for supplying the liquid contained in the liquid container to an outside, a valve mechanism disposed in the liquid supply port, a liquid flow passage allowing the liquid container and the liquid supply port to communicate with each other, and a differential pressure valve which is disposed in the liquid flow passage, which is normally urged to be a closed state, and which is changed from the closed state to an opened state when a differential pressure between a side of the liquid supply port and a side of the liquid containing chamber is equal to or more than a predetermined value, the method includes: removing at least a part of the valve mechanism; inserting a jig from the liquid supply port; forcibly opening the differential pressure valve using the jig against an urging force for urging the differential pressure valve to the closed state; and injecting the liquid from the liquid supply port to the liquid containing chamber through the liquid flow passage while forcibly maintaining the opened state of the differential pressure valve.