A view finder unit removably mounted on a single lens reflex camera body to enable the in-focus state of a picture-taking lens to be observed, the camera body having an opening portion provided to direct an imaging light beam projected by the picture-taking lens to the outside of the camera body, includes a housing removably mounted on the camera body and having an opening portion for directing the imaging light beam into the housing, said opening portion being coincident with the opening portion of the camera body, an optical system disposed in the housing to enable a first image formed by the imaging light beam to be observed through the opening portion of the housing, imaging means disposed in the housing for forming a second image substantially equivalent to the first image, in-focus detecting means including photoelectric detecting means disposed in the housing adjacent to the focal plane of the imaging means and producing an electrical signal representing the state of the second image in the focal plane, display means disposed in the housing and driven by the electrical signal of the in-focus detecting means to observably display the in-focus state of the picture-taking lens, and a battery chamber disposed in the housing for containing therein batteries which supply electrical energy to the in-focus detecting means and the display means.