Radiation characteristics of a plano-convex lens are studied using Maslov's method, stressing the evaluation of an electromagnetic field around a caustic region when a uniform plane wave is incident axially to the flat plate side. The field expressions which are valid near a focal region are derived in terms of finite integrals with a single variable. As a special case, an approximate expression for a weakly focusing system is derived and compared with a corresponding expression obtained using Kirchhoff-Huygens principle. Agreement between them is fairly good. Numerical computation for field distribution around a focal region is conducted, and the three-dimensional pictures for the results are presented to help understand the focusing behavior of the lens.