An optical observation system includes a spatial light modulator displaying a Fresnel type kinoform on a phase modulation plane, and modulating light L1 in phase to irradiate an observation object with modulated light L2, an imaging optical system imaging observation target light L3 from the observation object, an optical system moving mechanism moving the imaging optical system in an optical axis direction of the observation target light L3, and a control section controlling the optical system moving mechanism such that the focal position of the imaging optical system changes in response to a change in the light condensing position of the modulated light L2 by the Fresnel type kinoform.