To provide an image pickup optical device constituted so that an image having a viewing angle obtained only by an image forming optical system and an image having a viewing angle obtained by attaching an additional optical system can be simultaneously formed.

The image forming optical system 101, a luminous flux splitting element 102 and an enlargement optical system 103 having negative optical power are successively arranged on a first optical axis 104 on an object side. Then, luminous flux emitted from the optical system 101 is split to two luminous flux by the element 102. One split luminous flux becomes the luminous flux whose image is formed along a second optical axis 105 as it is. The other split luminous flux is made incident on the optical system 103 having the negative optical power and it becomes the luminous flux whose image having the small viewing angle is formed because the focal distance of the optical system 101 is enlarged as the whole system. A first image pickup element 201 and a second image pickup element 202 are arranged at the respective image forming positions of the split luminous flux so as to reproduce the luminous flux whose images are respectively formed as the images on the elements 201 and 202.