A phase-separated monolayer of a binary mixture of pyrene-labeled poly(isobutyl methacrylate) (PiBMA-Py) and perylene-labeled poly(octadecyl methacrylate) (PODMA-Pe) was investigated by the fluorescence scanning near-field optical microscope (SNOM); which provided optical images of samples with a high resolution beyond the diffraction limit, by using evanescent field from an aperture smaller than the wavelength of light. Each phase of the PiBMA-Py/PODMA-Pe was selectively imaged as a bright area by choosing the excitation wavelength (325- or 442-nm line of a He-Cd laser). The fluorescence SNOM enabled mapping of the excitation energy transfer efficiency on the monolayer plane. The energy transfer measurement revealed that the phase separation of the PiBMA-Py/PODMA-Pe blend at the air/water interface was almost completed by annealing for 60 min at 40 degrees C and the phase boundary had a width of a few hundred nanometers.