The invention relates to a focus detection optical system used with the so-called autofocus (AF) system mounted on single-lens reflex cameras (SLRs) or the like, and an imaging apparatus incorporating it. The focus detection optical system comprises at least n focus detection areas that are adjacent to or intersect each other on a predetermined imaging plane, where n.gtoreq.2. A re-imaging lens group comprises n+1 re-imaging lenses, A (n-1)th re-imaging lens and an nth re-imaging lens are a pair of re-imaging lenses that correspond to a (n-1)th focus detection area and are adjacent to each other. An nth re-imaging lens and a (n+1)th re-imaging lens are a pair of re-imaging lenses that correspond to the nth focus detection area and are adjacent to each other. The (n-1)th re-imaging lens and (n+1)th re-imaging lens are located at different positions.