An observation apparatus comprises an illuminating system for illuminating a material-to-be-observed, an elongated image transmitting member for fetching an image of the material-to-be-observed, and an image pickup device for picking up the image of the material-to-be-observed, which has been transmitted by the image transmitting member. The illuminating system is constituted such that illumination light having entered from an output end face of the image transmitting member into the image transmitting member may be irradiated to the material-to-be-observed through propagation inside of the image transmitting member. The slender portion containing the image transmitting member can thereby be made very thin and constituted as a cheap releasable unit, and a site to be observed can be illuminated accurately and efficiently.