A stereoscopic endoscope apparatus 1 includes: an image pickup device 13 configured to pick up optical images for a right eye and a left eye； a similarity degree determination portion 22 configured to receive input of an image signal for the right eye based on the optical image for the right eye and an image signal for the left eye based on the optical image for the left eye outputted from the image pickup device 13, compare the image for the right eye and the image for the left eye, and calculate a similarity degree between the image for the right eye and the image for the left eye； and an image signal output portion 23 configured to switch and output a three-dimensional image signal formed of the image signal for the right eye and the image signal for the left eye and a two-dimensional image signal formed of either one of the image signal for the right eye and the image signal for the left eye, based on the similarity degree calculated by the similarity degree determination portion 22.
