The purpose of the present invention is to reduce the size of an image pick-up lens unit. A part of a bundle of rays representing subject optical images is deflected vertically downward by a polarization prism, and is further deflected forwards by a total reflection mirror. A bundle of rays totally reflected by the total reflection mirror is split in three directions by a tri-directional splitting prism. The bundle of rays, which is split in three directions, is incident on a first optical-path-length-difference image pick-up element, a second optical-path-length-difference image pick-up element, and a phase-difference image pick-up element included in a phase-difference AF optical system. Auto focus (AF) is performed on the basis of the optical path length difference from signals obtained from the optical-path-length-difference image pick-up elements, and AF is performed on the basis of the phase difference from a signal obtained from the phase-difference image pick-up element. Since the phase-difference AF optical system is disposed so as to be parallel to the axis of the light forming the subject optical images, the size of the image pick-up lens unit can be reduced.