Impact vaporization causes the composition diversity of planetary atmospheres. An impact event induces a shock wave, which elevates pressures. The shock pressure field mainly determines the amount of vapor. It is well investigated by numerical simulations. However, the shock pressure field has not been determined analytically because of complexity of equation of state (EOS) for solid material. In this study, we carried out impact simulations and calculated the amount of vapor and the shock pressure field using ANEOS, which is a sophisticated solid EOS model. As a result, we found the difference of density dependence between the shock pressure and shock internal energy. Focusing on this difference, we could determine the shock pressure field analytically, and we analytically derived the amount of vapor consistent with simulation result.