An imaging optical system includes a first lens unit located at the most object-side position and at least one of lens units interposed between the first lens unit and an image. At least one of the lens units is moved along the optical axis. The first lens unit includes, in order from the object side, at least one lens with negative refracting power, a deformable mirror, and at least one lens with positive refracting power, and focusing is performed by the deformation of the deformable mirror.