Recently many kinds of high speed boats have been developed. The hydrofoil type is one of them. On considering the hydrofoil type, one important problem is interaction, for example, interaction between the main hull and the hydrofoil, the hydrofoil and the free surface etc. Here we consider the interaction between the free surface and a submerged lifting body by numerical simulation. The submerged lifting body is composed of a submerged body and a hydrofoil adapted to it. The numerical method is the Boundary Element Method (BEM) that was originally developed at Hiroshima University by Mori and Qi. We have modified the original method and have applied it to this case. We measured the lifting force and the pressure distribution on the hydrofoil in the NKK Tsu Ship Model Basin, and confirmed that BEM is an effective method for the interaction problem between the free surface and a submerged lifting body. By using this BEM, we considered the depth effect of the lifting force of the submerged lifting body, the optimum location of the adapted hydrofoil and the optimum submerged body shape.