We have performed precise measurements of the third-order dielectric constant in an antiferroelectric crystal MHPOCBC. In the vicinity of the SmA- SmCalpha* phase transition point an anomalous increase of it was observed. We have developed a Landau theory taking into account the pretransitional fluctuations and shown that the fluctuation can contribute to the third-order response. The anomalous behavior near the transition point was discussed on the basis of the theoretical results.