Based on the concept that there is a π phase jump property in the boundary wave, the diffraction of the plane wave and the spherical wave through an aperture is analyzed by means of wavefront diagram. This method can be used not only in the case where the aperture radius a is much larger than wavelength λ, but also in the case when a is comparable to λ. It is shown that many properties of diffraction in the Fresnel region can be obtained. As an example, a method to put the point of unit Fresnel number to infinity is suggested and demonstrated. The results of this report confirmed the existence of the π phase jump concept and its value in practical applicalions.