To provide a lens optical system made compact in the aspect of aberration by effectively using a diffraction grating.

This lens optical system is provided with a 1st group Gr1 having positive power, a 2nd group Gr2 having negative power, a 3rd group Gr3 having positive power and a 4th group Gr4 having positive power in turn from an object side. By changing an interval between the 1st and the 2nd groups Gr1 and Gr2 and an interval between the 3rd and the 4th groups Gr3 and Gr4, a zooming action is executed. The group Gr3 is provided with a joined lens and the joined lens is provided with the diffraction grating at a boundary surface (r14#). The curvature of the boundary surface (r14#) is different from those of the incident surface and the emitting surface (r13 and r15*) of the joined lens.