In order to find a clue to elucidation of the thresholdless response or V-shaped response observed in antiferroelectric smectic materials, an antiferroelectric phase of the Ising model with layered structure, which is known as metamagnetism is studied statically and dynamically under an external field. It is shown that the phase exhibiting a continuous change driven by the field is not necessarily a disordered phase, but such phenomenon occurs in the phase with long range order.