A diffraction grating produces first diffraction light in a symmetrical direction with respect to the 0-th diffraction light and the optical axis. Each light flux forms two flux interference patterns on a sample surface through first and second objective lenses. A sample is illuminated by spatially modulated illumination light. Fluorescence is generated on the sample by structured illumination light as excitation light. The fluorescence caught by the first objective lens forms a modulated image of the sample on a sample conjugate surface through an objective optical system including the first and second objective lenses. The modulated image is further modulated through the diffractive grating. Fluorescence from the further modulated image passes through a lens and a dichroic mirror, enters into a single light path of an observation optical system, passes through a fluorescent filter, and forms an enlarged image of the further modulated image through a lens.