To obtain a modifier for lignin-containing fibers capable of providing good color fastness to lightness of the lignin-containing fibers by hydrogen peroxide bleaching which has hitherto been regarded as unsuitable to bleaching of the lignin-containing fibers without using chlorine bleaching having environment problems and to provide a method for hydrogen peroxide bleaching using the modifier.

This modifier for the lignin-containing fibers includes a quaternary ammonium salt type surfactant represented by the following general formula (1) (wherein, R1, R2, R3, and R4 are each a group selected independently from the group consisting of 1-18C alkyl groups; one of them may contain a benzyl group or a hydroxy group; and X denotes a halogen atom and a 1-18C alkylsulfuric acid group). The method for hydrogen peroxide bleaching of the lignin-containing fibers uses the modifier.