This letter presents an antennadiagnosis method using infinitesimal dipole modeling. For antenna diagnosis, it is important to estimate the current distribution of the antenna aperture. By using the measured near-field data of the antenna, the infinitesimal dipole current distribution that can generate the same near-field measurement result is extracted. This current distribution can be considered as a back-projection image on the antenna aperture. Using the infinitesimal dipole modeling, accurate antenna diagnosis is possible even by using only the measured near-field which is limited over the antenna aperture. We compared the results of the conventional antenna diagnosis method based on Fourier transform and the proposed method for the near-field measurement results of the standard gain horn antenna. Although the limited near-field measurement over the antenna aperture, the back-projection image for the antenna diagnosis of the proposed method has a very high resolution.