PURPOSE: To obtain a dyed product good in fastness by dying cellulose with a water-soluble sulfur dye without a reductant, an added salt and an oxidant wherein the cellulose has been grafted with an N-contg. basic polymer.

CONSTITUTION: Cellulose which has been grafted with an N-contg. basic polymer obtained by polymerization of an N-contg. basic monomer is dyed with a water- soluble sulfur dye, particularly a water-soluble thiosulfate deriv., without addition of a reductant. As the N-contg. basic monomer, dimethyl-diallyl ammonium chloride and vinyl piridine, etc., are preferable. Grafting is carried out by introducing, for example, the cellulose into a polymerization vessel together with a monomer soln. and polymerizing them. Grafting is preferably carried out in a mixed soln. of water and a water-miscible solvent such as alcohol at pH 3-8.5 and at a temp. of 65-85°C.