Phase diagram of the ANNNI model with the third neighbour interaction is obtained to clarify the ferrielectric phases appearing in the sequences of phase transitions of certain ferroelectric smectic liquid crystals. While various mesophases are ferrielectric, those with wave numberq= 3/8 and 3/10 come out to exhibit the second order phase transition from antiferroelectric phase to ferrielectric one with decreasing temperature. The phases withq= 4/13 and 4/11 are the candidates for the ferrielectric phases FIHand FILobserved just above and below the SmCy*, respectively.