PURPOSE: To uniformly and fast dye leather materials with excellent affinity of dyes and clear color contrast by incorporating a polycondensation product of specific amines and carboxylic acids into a dyeing bath for leather dyeing with anionic dyestuffs.

CONSTITUTION: The leather is uniformly dyed with dense and clear color contrast by adding into the bath containing an anionic dyestuff a polyaminoamide resin obtained by condensation reaction of one of diamines shown by formula I (wherein R1 and R3 are each H, a non-substituted or hydroxyl-substituted 1-10C (preferably 1-4C) alkyl or radical obtained by addition reaction of 2-4 ethylene oxides and/or butylene oxides; R2 is a 2-9C alkylene or bivalent radical shown by formulae II, III or IV; (n) is 1 or 2; (m) and (l) are each 0 or 1; (p) is 0, 1, 2 or 3) and piperazine derivatives shown by formula V (wherein R4 and R5 are each H, a 1-4C alkyl or the like; R6 is a 1-4C alkyl; R7 is a 1-4C alkylene) with a 4-12C aliphatic saturated dicarboxylic acid.