A defoaming agent including a polymer, the polymer including a repeating unit represented by the following general formula (1); and a repeating unit represented by the following general formula (3): wherein in the general formula (1), X1 is a repeating unit obtainable by polymerization of a first ethylenic unsaturated group; Y1 is a side chain including a linear or branched polysiloxane structure, the polysiloxane structure having a polymerization degree of 5 to 300; and Z1 is a linking group linking the repeating unit X1 and the side chain Y1; wherein in the general formula (3), X2 is a repeating unit obtainable by polymerization of a second ethylenic unsaturated group, Y2 is a C1-30 hydrocarbyl side chain; and Z2 is a linking group linking the repeating unit X2 and the side chain Y2.