To provide a deinking agent for recycling waste paper capable of producing high-quality deinked pulp with extremely little residual ink.

The deinking agent includes a polymer (a) obtained by copolymerizing one or two or more monomers (I) {wherein, R1, R2, R3, and R4 represent each independently a hydrogen atom or a 1-22C hydrocarbon group; X represents -(CH2)bO- or -COO-; AO represents a 2-4C oxyalkylene group; a is the average addition molar number of an alkylene oxide, and represents a number of 1 to 200; and b represents an integer of 1 to 20} and fumaric acid as essential components.