A device for continuously supplying ink under constant pressure comprising an ink storage tank (3) and an ink feeding pipe connecting an ink chamber with a print head. The ink storage tank includes a container having a relatively large ink chamber which has a gas passage connected with the atmosphere. Inside the ink storage tank is a first chamber (33) and a second chamber (35). A gas inlet port (31) connected with the atmosphere is provided inside the first chamber. At the lower part of the partition between the two chambers, there is provided a gas-liquid exchange entryway (37) that allows gas in the first chamber to enter into the second chamber and allows ink in the second chamber to flow into the first chamber. Utilizing an equilibrium principle of gas pressure, constant pressure processes can be designed and manufactured.