A booster system for increasing pressure of an object gas includes: a first compression unit that compresses the object gas to intermediate pressure equal to or higher than the critical pressure and lower than the target pressure and generates an intermediate supercritical fluid; a cooling unit that cools the intermediate supercritical fluid with a cooling medium and generates an intermediate supercritical pressure liquid; a liquid extracting and pressure reducing unit that extracts a part of the intermediate supercritical pressure liquid; a flow regulating valve that regulates a flow rate of the extracted part of the intermediate supercritical pressure liquid; a second compression unit that increases pressure of the rest of the intermediate supercritical pressure liquid to be equal to or higher than the target pressure; and a pressure sensor that detects pressure of the intermediate supercritical pressure liquid.