A monoazo compound, or a salt thereof, of the formula, ##STR1## wherein W is a hydrogen atom or a carbamoyl group, R.sub.1 is a lower alkyl group, R.sub.2 and R.sub.3 are independently a hydrogen atom or a lower alkyl group, A is an unsubstituted or substituted phenylene or naphthylene group, Y is --SO.sub.2 CH.dbd.CH.sub.2 or --SO.sub.2 CH.sub.2 CH.sub.2 Z, in which Z is a group capable of being split by the action of an alkali, and X is an aromatic or aliphatic group substituted with one or two members selected from sulfonic acid group and carboxylic acid group, which is useful for dyeing fiber materials to give dyed products of a characteristic yellow color excellent in various fastness properties.