To provide an observation device such as binoculars, a monocular and a spotting scope by which observation by naked eyes is simultaneously performed with the display of a picked-up image by a camera.

The observation device for observing an image formed by an objective group L1 through an eyepiece group L2 is provided with a half mirror 21 arranged between the focal plane FP of the objective group L1 and the objective group L1 in the optical path of the objective group L1, a negative lens 23 arranged in a branched optical path branched by the mirror 21, and an attaching/detaching part 17 attaching/detaching a photographic optical system on the emitting side of the negative lens 23. An electronic camera 31 capable of picking up an image through an objective optical system is formed attachably/detachably through the edge part of an image pickup lens 33 by the attaching/ detaching part 17 of the photographic optical system.