[en] The observation of X-ray is developed in the energy band less than 1 keV, and the sensitive counters with excellent energy resolving power are required. The proportional counter (PC) which has the resolving power of about 20% for 5.9 keV is usable if strong line spectra exist, but for the line spectra less than 1 keV, the counter with better energy resolving power is necessary especially for the observation of 0-7 (0.57 keV), 0-8 (0.65 keV), Fe-17 (0.82 keV) and Ne-9 (0.92 keV). Fluorescence proportional counters (SPC) are available for the special observation of ultrasoft X-ray. The observation result with PC using LEINAX-4 and the expected result if SPC is used are shown assuming the energy resolving power of the SPC as 8%. The relation between actual energy spectra and observed ones with PC or SPC is analyzed, and the fundamental formula is solved by the iteration method. This iteration method for both PC and SPC is explained regarding the characteristics of convergence, division of energy band and the energy resolving power comparing to the observed result. In the energy band less than 0.1 keV, the micro channel plate (MCP) began to be employed, for efficiency becomes good and is about 10% in this energy band, but it is necessary to eliminate the influence of strong He 2 Lα (304 A) in the observation. But SPC is available down to the region of good efficiency with MCP and plays major role in the observation of ultrasoft X-ray. The effect of He 2 Lα (304 A) on PC and SPC was compared. (Nakai, Y.)