A stereoscopic imaging optical system having a function to adjust a stereo base, an imaging device that includes the stereoscopic imaging optical system, and a camera that includes the imaging device, are provided. The stereoscopic imaging optical system includes two imaging lens systems arranged in parallel, and two diaphragms arranged in the imaging lens systems, respectively. At least one of the diaphragms is decentered with respect to an optical axis to adjust the stereo base. Further, a condition (1.8≦SBmax/(fT×tan(ωT))≦45, where SBmax is a maximum value of the stereo base, fT is a focal length of the optical system at a telephoto limit, and ωT is a half view angle (°) at the telephoto limit) is satisfied.