an imaging optical system includes first to third lens groups , and performs focusing by moving the second lens group , to satisfy the following conditional expressions : 0 . 8 & lt ; 1 gr — fl / fl & lt ; 1 . 2 , 0 . 6 & lt ;| 2 gr — fl / fl |& lt ; 2 . 5 , 0 . 6 & lt ; 3 gr — fl / fl & lt ; 1 . 8 0 . 8 & lt ; 1 gr — r — nop / 1 gr — thi & lt ; 1 . 1 , and − 0 . 6 & lt ; 2 gr — r — nop / 2 gr — thi & lt ; 1 . 6 where fl , 1gr_fl , 2gr_fl , and 3gr_fl represent focal lengths of the entire system , and the first to third lens groups , respectively , 1gr_r_nop represents a distance from the outermost surface of the first lens group on the object side to a rear principal point of the first lens group , 1gr_thi represents a total length of the first lens group , 2gr_r_nop represents a distance from the outermost surface of the second lens group on the object side to a rear principal point of the second lens group , and 2gr_thi represents a total length of the second lens group .