Unwinding process of Smectic C-alpha* phase (SmC alpha*) to smectic C phase (SmC) in an electric field looks similar to a transition from chiral smectic C phase (SmC*) to SmC which is interpreted as a soliton condensation. As a pitch of the helical structure is quite short in the former, a discrete description is required and the soliton accompanying with should be a discrete type, while in the latter, the soliton is a kink of sine-Gordon equation. Under the condition of constant tilt at SmC alpha*, it has been elucidated that for the helical pitch larger than four-layer the transition is second order and a wave number versus field relation makes a devil's staircase. Here, a free energy curve for the wave number is proved to be non-differentiable at any rational wave number, monotonous and convex, corresponding to the devil's staircase structure of the wave number. This non-analytic property contrasts with the case of continuous description in SmC*, where the free energy curve is analytic. In the framework of the present model, a change of apparent optical axis and switching current is calculated, which are compared with experimental results reported so far.