An optical apparatus used for a 3D image pickup apparatus for taking two subject images having a disparity by using two lens apparatuses, each of which is directly connectable to an image pickup apparatus, and one image pickup apparatus, the optical apparatus including: a first attaching unit for detachably attaching a first lens apparatus; a second attaching unit for detachably attaching a second lens apparatus; a camera attaching unit for detachably attaching the image pickup apparatus, the image pickup apparatus including an image pickup portion; and a switch unit for alternately switching light rays from the first and second lens apparatuses to guide the light ray to the image pickup apparatus in a state that the first and second lens apparatuses and the image pickup apparatus are connected to the optical apparatus. Intermediate images are formed in the optical apparatus by the first and second lens apparatuses.