An imaging optical lens system includes six lens elements, the six lens elements being, in order from an object side to an image side, a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element and a sixth lens element. The first lens element has positive refractive power. The third lens element has an image-side surface being concave in a paraxial region thereof. The fifth lens element has an object-side surface and an image-side surface being both aspheric. The sixth lens element has an image-side surface being concave in a paraxial region thereof, wherein an object-side surface and the image-side surface of the sixth lens element are both aspheric, and the image-side surface of the sixth lens element includes at least one convex critical point in an off-axial region thereof.