It is known that most of amphiphilc molecules show a phase transition from monolayer to trilayer, not bilayer, when they are compressed at an air-water interface. However, the reason why they form a trilayer is not thoroughly investigated. In this Monte Carlo simulation study, we suggest that a monolayer-trilayer phase transition is due to water molecules accompanied by the amphiphilic molecule in the upper part of the layer. An amphiphilic molecule was modeled as consisting of two parts, hydrophilic and hydrophobic parts, and the amount of water accompanied by the amphiphilic molecule was assumed to be exponentially decaying with the height. Lennard-Jones interaction was used as interactions between molecules. From the simulation results, it was found that the odd-numbered layer is stable because of the hydrophobic effect and surface tension of the water. Further simulations show that as more water is accompanied by the molecules in the higher floor of the layer the odd-numbered layer becomes more stable and so the internal ordering of molecules does. The calculated π-A isotherm is consistent with previous experimental results and suggests that the transition is first-order.