To prevent incidence of strong external light into a photodetector and an imaging device while a lid is opened, so as to thereby perform accurate photodetection and imaging operations without receiving the influence of external light. An observation apparatus comprising: a stage on which a sample serving as an observation target is placed; a detection unit which detects light from the sample; an imaging optical system which projects the light from the sample onto the detection unit; a light shielding member which covers the whole or a part thereof; an opening provided in the light shielding member; a lid which opens/closes the opening; an open/close detection unit which detects the open/close state of the lid; a dimmer which limits light incident into the detection unit; and a controller which operates the dimmer to reduce the amount of the light incident into the detection unit, when the open/close detection unit detects that the lid has been opened.