PROBLEM TO BE SOLVED: To provide a leather-dyeing auxiliary which can allow dyes to be penetrated and adsorbed inside the leather and gives optionally dyed leather products without any adverse effect on the flexibility of the leather itself and can improve the deep dyeability to the leather and can avoid the dyes from cracking. SOLUTION: This leather-dyeing assistant auxiliary comprises a water-based liquid of cationic polyurethane resin prepared by dispersing or dissolving one or more kinds of cationic polyurethane resins produced by neutralizing a part or the whole of the tertiary amino groups in the molecule of polyurethane resulting from the reaction between (A) a polyester-polyol having at least two of active hydrogen atoms reactive with isocyanate groups and contains ester groups with a weight-average molecular weight of 500-5,000, and (B) a diol bearing a tertiary amino group with a weight-average molecular weight of =200, and (C) an organic isocyanate bearing at least two isocyanate groups.
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