Block-spin model (or ANNNI model) characterized by frustration due to competing interaction and 4-state model in which each chiral molecule takes four states of molecular orientation are considered in order to clarify a mechanism of appearance of intermediate phases in antiferroelectric smectics. It is concluded that the intermediate phases are caused not only by the frustrated interactions but also by unsymmetrical interaction energy in adjacent block-spin or in adjacent chiral molecule along layer normal.