Some problems in a near-field scanning optical microscope (NSOM) system for observing biological living specimens is discussed. The NSOM system works based on the principle of nano-optics and near-field optics. Propagating characteristics of evanescent wave field, the effects of absorption, polarization changes, fluorescence, optical density, refractive index, luminescence and incident angle are analyzed. Methods eliminating pseudo-image in specimens are proposed. NSOM will be employed to conduct investigation of single molecule of living biological specimen and make precision metrology researches in nanometer scale.