PROBLEM TO BE SOLVED: To optically observe action of a brain or the like of a small animal such as a mouse while permitting free motion of the small animal.SOLUTION: An optical observation system 1 includes a transportable device 2 fixed on a test object A and allowed to be moved together with the motion of the test object A and an external device 3 arranged on the outside of the test object A. The transportable device 2 includes: a light source; an illumination optical system for irradiating an observation target portion of the test object A with light from the light source; a detection optical system for guiding light from the observation target portion; a photodetector for detecting the light guided from the detection optical system and converting the light into an electric signal; and a transmitter 5 for wirelessly transmitting the electric signal outputted from the photodetector. The external device 3 includes a receiver 20 for receiving the electric signal transmitted from the transportable device 2 and a signal processing part 21 for processing the electric signal received by the receiver 20.