Provided is a stereoscopic image-capturing objective optical system (1) including a first prism pair (3) that converts beams emitted from a single subject and having two substantially parallel optical axes arranged side-by-side in one direction with a certain distance therebetween into beams arranged side-by-side in a direction intersecting the aforementioned side-by-side direction with a certain distance therebetween； and a second prism pair (5) that performs conversion to reduce the distance between the optical axes of the two beams converted by the first prism pair (3) and that has exit surfaces arranged side-by-side in a direction perpendicular to the side-by-side arrangement direction before entering the first prism pair (3). The first prism pair includes a first parallelogram prism that reflects, twice, the beam containing one of the two optical axes in a first plane containing said one of the optical axes, and a second parallelogram prism that reflects, twice, the beam containing the other of the two optical axes in a second plane containing the other optical axis and parallel to the first plane with a certain distance therebetween.
