To provide an optical microscope and a method for judging viability or measuring cell activity without giving adverse influences on cells.

The optical microscope 3 is equipped with: a diode laser 2 (light source) which emits probe light 9 (a light beam); an illumination port 4 to guide probe light 9 to an objective lens; an objective lens 12 (condensing means) to converge the probe light 9 near the cell 7 being the object of observation; a lens-lens system 18 to condense the probe light 9 transmitting through the cells 7; and a deflection detector 10 to detect deflection of the probe light. In the the above optical microscope 3, since the probe light 9 is deflected by density gradation near the cell membrane of living cells, viability of cells and cell activity can rapidly and easily be judged and measured by measuring the deflection.