To obtain a composition for dyeing a keratin fiber such as human hair, showing a good resistance to attacking factors to the hair, not having selectivity and capable of dyeing more potently by including a specific cationic direct dye and a thickening polymer in a medium suitable for the dyeing.

This dyeing composition for a keratin fiber containing a cationic direct dye and a thickening polymer is obtained by including, in a medium suitable for the dyeing, (A) 0.001-10 wt.% cationic direct dye based preferably on the total weight of the composition and selected from compounds of (i) a compound of formula I [D is N or CH group; R1, R2 are each H, a heterocyclic ring or the like; R3, R'3 are each H, Cl or the like; X- is chloride or the like; A is a group of formula II (R4 is a 1-4C alkyl) or the like] and (ii) a compound of formula III (R6 is H or a 1-4C alkyl; R7 is a heterocyclic ring or the like; R8, R9 are each H, bromine or the like; B is a group of formula IV (R10 is a 1-4C alkyl) or the like] and (B) at least one kind of a thickening polymer having at least one sugar unit (preferably a nonionic guar gum, etc.).