To enable a cellulose material to be dyed in blue with a very high lightfastness by using a specific new disazo dye.

This dye is represented by formula I [wherein the sulfo group is bonded to the 6- or 7-position; R is a benzoyl or naphthoyl group substd. by a 1-4C alkyl, a 1-4C alkoxy, F, Cl, Br, COOH, SO3H, or NO2 group or is a 1,3,5-triazinyl group susbtd. at the 4- or 6-position by an OH, NH2, NHC2H4 OH, N(CH3)C2H4OH, or N(CH2OH)2 group or by a group represented by formula II; and X is SO3H or COOH] and is used for dyeing the surface of paper produced from bleached or unbleached cellulose derived from various materials. When used for the surface treatment, the dye is applied after the sheet is formed. The dye can be used as a solid dye preparation in the form of powder or granules. A liq. preparation of the dye generally contains it in a concn. of 10-40wt.%.