An imaging lens system includes six non-cemented lens elements with refractive power, 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 second lens element has refractive power. The third lens element has positive refractive power. The fourth lens element has refractive power. The fifth lens element has refractive power, wherein both of the surfaces thereof are aspheric. The sixth lens element with refractive power has a concave image-side surface in a paraxial region thereof, wherein the image-side surface has at least one convex shape in an off-axis region thereof, and both of the surfaces thereof are aspheric. The imaging lens system has a total of six lens elements with refractive power.