To obtain a scarlet dyestuff excellent in dyeing property for cellulose fibers and provide a dyeing method for cellulose fibers using the same.

A monoazo compound of formula (1) (wherein (m) and (n) are each an integer of 0 or 1; R1 is hydrogen or an alkyl group; R2 is hydrogen, an alkyl group, an alkoxy group or a sulfone group; Z1 is a heterocyclic reactive group containing no vinylsulfone based reactive group, a vinylsulfone based reactive group containing a triazine ring, a vinylsulfone based reactive group containing an acid group or a vinylsulfone based reactive group; U is an amino residue group without fiber reactivity).