We investigated the bilayer interdigitation of diacylphosphatidylcholines (CnPCs) with long acyl chains (n = 19-22) under high pressure. The temperature-pressure phase diagrams of the CnPC bilayers were constructed by the methods of differential scanning calorimetry and high-pressure light transmittance. We confirmed the bilayer interdigitation for CnPCs (n = 19-21) under high pressure. On the other hand, the C22PC bilayer showed peculiar behavior: two transitions were observed under atmospheric pressure while one transition under high pressure. To identify the low-temperature phase of the C22PC bilayer, we performed small angle neutron scattering measurements under atmospheric pressure and fluorescence measurements under high pressure. From these results, the phase was identified as the interdigitated gel (LβI) phase. Considering our previous results, it turned out that the acyl-chain length of CnPC that exhibits the pressure-induced interdigitation is limited within n = 14-21. The present results definitely indicate that the bilayer interdigitation is governed by the hydrophobic interaction as well as the hydrophilic interaction of the PC molecule in the bilayer.