An apparatus (1) for mixing a paste material with gas includes: a mixing part (2) that mixes the paste material with the gas using a piston pump (10); and a static mixer (3) that is connected to the mixing part to stir a mixture obtained by mixing the paste material with the gas in the mixing part. The static mixer includes one or a plurality of stirring sections through which the mixture passes, the stirring section has a shape that allows a flow of the mixture passing through the stirring section to be stirred. A ratio of a volume of the piston pump to a volume of at least one of the stirring sections of the static mixer is within a range from 1:0.2 to 1:5.