An air pressure regulating device comprises an expansion tank having air stored therein and having an air injection/discharge port at an outside thereof; and a pressure regulation unit provided with a three-way ball valve connected to the air injection/discharge port, a pressure gauge which is installed in any one of flow paths of the three-way ball valve to check an internal pressure of the expansion tank, and a valve core which is installed in a flow path other than the flow path in which the pressure gauge is installed to be capable of injecting or discharging air into or from the expansion tank so as to be capable of controlling the internal pressure of the expansion tank.