The importance of the interlayer molecular interaction to stabilize the antiferroelectric smectic C_A^* (SmC_A^*) phase is shown using dimer molecules in which chiral mesogenic units are connected by a polymethylene chain. According to the conformational effect of the polymethylene chain, two mesogenic units tend to align parallel or anticlinically depending on the carbon number, even or odd, respectively. In a dimer molecule with a longer chain length, however, the other interaction overcomes the conformational effect and stabilizes the SmCA^* phaec. This observation suggests the importance of the electric, possibly the dipole-dipole, interaction between mesogens in SmCA^*.