A thiazine derivative, which forms a color on contact with an electron accepting material, represented by the formula (II): ##STR1## wherein A.sub.1 and A.sub.2, which may be the same or different, each represents an aryl group or a heterocyclic group and A.sub.1 and A.sub.2 may combine together to form a heterocyclic ring or a fluorene ring, the ring B represents an aromatic hydrocarbon ring or a heterocyclic ring, and Y represents a hydrogen atom, an aliphatic group, an aryl group, a heterocyclic group, an amino group, an amido group, an oxy group or a thio group, but at least one of A.sub.1 and A.sub.2 or the ring formed by the combination of A.sub.1 and A.sub.2 represents an electron donating aryl group or an electron donating heterocyclic group, and a process for preparing the thiazine derivatives represented by the formula (II) by oxidizing a thioamide derivative represented by the formula (I): ##STR2## wherein A.sub.1, A.sub.2, B and Y are as above described.