A method for mixing gas into a high-viscosity material comprising each of the steps of: pumping the high-viscosity material from a tank that stores the high-viscosity material by a pressure pump, the high-viscosity material flowing through a conduit (50, 47, 52); closing, using a discharge valve, a discharge port provided so as to make a cylinder communicate with the conduit through which the high-viscosity material flows; moving away a piston arranged inside the cylinder from the discharge port to form a cylinder space having a predetermined volume in the cylinder; filling the cylinder space with gas having a predetermined pressure, wherein the gas is supplied to the cylinder space through a suction valve by a gas compressor, wherein the suction valve controls the gas supply to the cylinder space; moving the piston toward the discharge port to compress the gas; and opening, using the discharge valve, the discharge port to mix the compressed gas into the high-viscosity material that flows through the conduit and sending the high-viscosity material into which the gas has been mixed to a nozzle through the conduit, wherein the gas is dispersed and stirred into the high viscosity material while flowing through the conduit (47, 52); wherein the series of steps from closing the discharge port to opening the discharge port is repeated each time when a predetermined amount of the high-viscosity material flows, thereby continuously supplying to the nozzle the high-viscosity material into which the gas has been mixed, the nozzle being attached to a tip of a gun; wherein the nozzle is for applying to a work piece the high-viscosity material into which the gas has been mixed and can arbitrarily discharge the high viscosity material; and wherein a foam ratio of the high-viscosity material discharged from the nozzle is controlled by adjusting at least any one of the predetermined amount of the high-viscosity material, the predetermined volume of the cylinder space, and the predetermined pressure of the gas. pumping the high-viscosity material from a tank that stores the high-viscosity material by a pressure pump, the high-viscosity material flowing through a conduit (50, 47, 52); closing, using a discharge valve, a discharge port provided so as to make a cylinder communicate with the conduit through which the high-viscosity material flows; moving away a piston arranged inside the cylinder from the discharge port to form a cylinder space having a predetermined volume in the cylinder; filling the cylinder space with gas having a predetermined pressure, wherein the gas is supplied to the cylinder space through a suction valve by a gas compressor, wherein the suction valve controls the gas supply to the cylinder space; moving the piston toward the discharge port to compress the gas; and opening, using the discharge valve, the discharge port to mix the compressed gas into the high-viscosity material that flows through the conduit and sending the high-viscosity material into which the gas has been mixed to a nozzle through the conduit, wherein the gas is dispersed and stirred into the high viscosity material while flowing through the conduit (47, 52); wherein the series of steps from closing the discharge port to opening the discharge port is repeated each time when a predetermined amount of the high-viscosity material flows, thereby continuously supplying to the nozzle the high-viscosity material into which the gas has been mixed, the nozzle being attached to a tip of a gun; wherein the nozzle is for applying to a work piece the high-viscosity material into which the gas has been mixed and can arbitrarily discharge the high viscosity material; and wherein a foam ratio of the high-viscosity material discharged from the nozzle is controlled by adjusting at least any one of the predetermined amount of the high-viscosity material, the predetermined volume of the cylinder space, and the predetermined pressure of the gas.