We consider syntactic congruences of some codes. As a main result, for an infix code L, it is proved that the following (i) and (ii) are equivalent and that (iii) implies (i), where PL is the syntactic congruence of L. (i) L is a PL2-class. (ii) Lm is a PLk-class, for given two integers m and k with 1 m k. (iii)L* is a PL*-class. Next we show that every (i), (ii) and (iii) holds for a strongly infix code L. Moreover we consider properties of syntactic conguences of a residue W(L) for a strongly outfix code L.