To provide a lens optical system capable of reducing the size of an image display apparatus such as a head-up display.

A lens optical system includes in order from an object side: a first convex lens group 20 having a plurality of first element convex lenses 21 planarly arranged therein; a second convex lens group 22 having a plurality of second element convex lenses 23 planarly arranged therein; and a third convex lens group 24 having a plurality of third element convex lenses 25 planarly arranged therein. The lens optical system is configured so that lens diameters and lens pitches of the respective convex lenses are set to be larger in order of the first convex lens group 20, the second convex lens group 22 and the third convex lens group 24, and a distance between the second convex lens group and the third convex lens group is set to be longer than a distance between the first convex lens group and the second convex lens group.