A scanning optical microscope includes a light source; a first beam splitter that splits light into irradiation light and reference light; a first objective lens that converges the irradiation light on a sample and receives signal light; a second beam splitter that splits the signal light off from an optical path; a pin hole positioned on an optical path of the split signal light at a position optically conjugate with an image-forming point of the first objective lens; a condenser lens that converges the split signal light on the pin hole; a phase plate that outputs first light including at least four firstly-split beams having different phases; a third beam splitter that multiplexes the first light and second light to generate interfering light including at least four secondly-split beams having different phases; and a light detecting element that receives the interfering light and outputs at least four electric signals.