An imaging lens system includes, in order from an object side to an image side, a first lens element, a second lens element, a third lens element and a fourth lens element. The first lens element with negative refractive power has an image-side surface being concave in a paraxial region thereof. The second lens element has negative refractive power. The third lens element has positive refractive power. The fourth lens element has negative refractive power. The imaging lens system has a total of four lens elements, and the imaging lens system further includes an aperture stop disposed between the second lens element and the third lens element.