To provide a method for bulk dyeing of plastics.

The method for bulk dyeing of plastics is effected by using a dyestuff of formula I, wherein Y is a divalent group selected from the group consisting of a substituted or unsubstituted arylene group and a substituted or unsubstituted alkylene group in which the alkylene group may be optionally interrupted by a hetero atom, for example, such as O or S; R1 and R1' are each independently an electron-withdrawing group, preferably a cyano or alkoxycarbonyl group, particularly a 1-4C alkoxycarbonyl group; R2, R2', R3 and R3' are each independently a hydrogen atom, an alkyl group, particularly a 1-4C alkyl group, CF3 or a halogen atom, preferably F or Cl; and R4 and R4' are each independently an unsubstituted or substituted alkyl group, preferably a 1-6C alkyl group, or a 7-12C aralkyl group, preferably a benzyl group.