A variable air pressure regulating device for an expansion tank, comprising an expansion tank provided with a rubber bag therein, an air injection/discharge port at an outer side thereof, and a drain on a bottom thereof, the drain capable of discharging water to be leaked when the rubber bag is burst; and a pressure regulation unit capable of regulating an internal pressure of the expansion tank; characterized in that the pressure regulation unit is connected to the air injection/discharge port and includes: a three-way ball valve which is provided with an upper flow path, a lower flow path, and a side flow path formed in three directions of upward, downward, and sideward and provided with a flow path setting member capable of selectively setting a flow path at a center where the three flow paths meet; a pressure gauge installed in the upper flow path of the three-way ball valve to check the internal pressure of the expansion tank; and a valve core installed in the lower flow path of the three-way ball valve to be capable of injecting or discharging air into or from the expansion tank, wherein the side flow path is coupled to the air injection/discharge port; and the flow path setting member includes: a valve having a shape of a ball; a variable flow path in which two flow paths are formed in the valve at 90 degrees in a penetrating manner; and a lever by which the valve can be operated, wherein the setting member is configured such that when the lever is operated vertically downward, the variable flow path forms a flow path toward the expansion tank and the pressure gauge, so that the pressure of the expansion tank can be checked through the pressure gauge, and when the lever is operated 90 degrees to the left, the variable flow path forms a flow path toward the expansion tank and the valve core, so that air can be injected into the expansion tank, and when the lever is operated toward a direction of 45 degrees, the flow path is set so that the variable flow path connects all of the expansion tank, the valve core, and the pressure gauge. an expansion tank provided with a rubber bag therein, an air injection/discharge port at an outer side thereof, and a drain on a bottom thereof, the drain capable of discharging water to be leaked when the rubber bag is burst; and a pressure regulation unit capable of regulating an internal pressure of the expansion tank; characterized in that the pressure regulation unit is connected to the air injection/discharge port and includes: a three-way ball valve which is provided with an upper flow path, a lower flow path, and a side flow path formed in three directions of upward, downward, and sideward and provided with a flow path setting member capable of selectively setting a flow path at a center where the three flow paths meet; a pressure gauge installed in the upper flow path of the three-way ball valve to check the internal pressure of the expansion tank; and a valve core installed in the lower flow path of the three-way ball valve to be capable of injecting or discharging air into or from the expansion tank, wherein the side flow path is coupled to the air injection/discharge port; and the flow path setting member includes: a valve having a shape of a ball; a variable flow path in which two flow paths are formed in the valve at 90 degrees in a penetrating manner; and a lever by which the valve can be operated, wherein the setting member is configured such that when the lever is operated vertically downward, the variable flow path forms a flow path toward the expansion tank and the pressure gauge, so that the pressure of the expansion tank can be checked through the pressure gauge, and when the lever is operated 90 degrees to the left, the variable flow path forms a flow path toward the expansion tank and the valve core, so that air can be injected into the expansion tank, and when the lever is operated toward a direction of 45 degrees, the flow path is set so that the variable flow path connects all of the expansion tank, the valve core, and the pressure gauge. a three-way ball valve which is provided with an upper flow path, a lower flow path, and a side flow path formed in three directions of upward, downward, and sideward and provided with a flow path setting member capable of selectively setting a flow path at a center where the three flow paths meet; a pressure gauge installed in the upper flow path of the three-way ball valve to check the internal pressure of the expansion tank; and a valve core installed in the lower flow path of the three-way ball valve to be capable of injecting or discharging air into or from the expansion tank, wherein the side flow path is coupled to the air injection/discharge port; and the flow path setting member includes: a valve having a shape of a ball; a variable flow path in which two flow paths are formed in the valve at 90 degrees in a penetrating manner; and a lever by which the valve can be operated, wherein the setting member is configured such that when the lever is operated vertically downward, the variable flow path forms a flow path toward the expansion tank and the pressure gauge, so that the pressure of the expansion tank can be checked through the pressure gauge, and when the lever is operated 90 degrees to the left, the variable flow path forms a flow path toward the expansion tank and the valve core, so that air can be injected into the expansion tank, and when the lever is operated toward a direction of 45 degrees, the flow path is set so that the variable flow path connects all of the expansion tank, the valve core, and the pressure gauge.