The imaging optical system makes magnification conjugate side and reduction conjugate side predetermined positions conjugate to each other and forms an intermediate image conjugate to both the predetermined positions. The optical system includes a magnification side positive lens as a first lens disposed on the magnification conjugate side further than an intermediate imaging position, a reduction side positive lens as a third lens disposed on the reduction conjugate side further than the intermediate imaging position, and a negative lens as a second lens disposed between these positive lenses. Conditions of 5