The method of approaching the near-field diffraction patterns by sampling theorem is proposed. In the near-field diffraction, the aperture is well-defined space-function, and approximately so does the diffraction field distribution. The diffraction field can be restored with rather high precision by sampling function in its spatial spectrum. The approximate value of the spectrum bandwidth of the diffraction field is the region in which the spectrum amplitude is larger than 1% of the maximum of its center spectrum. The spatial spectrum is sampled in this bandwidth. The sampling space is proportional to the bandwidth and inversely to the space-bandwidth product of the aperture function. The calculation results of the near-field diffraction patterns of slit and circle apertures with the size smaller than the wavelength are coincided with other theoretical and experimental results. The sampling method has advantages of direct perceivable and forthright.