A deinking agent containing a compound represented by the general formula (I): R O-(PO)m-(EO)n-(XO)k-R wherein R represents C12-C22 alkyl or alkenyl; R represents hydrogen or C2-C24 acyl; PO, EO and XO represent, respectively, a propylene oxide unit, an ethylene oxide unit and a C3 or higher alkylene oxide unit; m is 1 to 8, n is 10 to 40 and k is such a number as to give polymerized XO units wherein the total number of the carbon atoms is 3 to 120; and wherein PO and EO form block addition copolymer chains. This agent is used for deinking waste paper particularly by flotation to give a high-quality regenerated pulp having a high whiteness and a reduced residual ink content and enable stabilized operation.