A photographing 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 has positive refractive power. The second lens element has negative refractive power. The third lens element having positive refractive power has an image-side surface being concave, and an object-side surface and the image-side surface thereof being aspheric. The fourth lens element has an object-side surface being concave, and the object-side surface and an image-side surface thereof being aspheric. The photographing lens system further includes an aperture stop, and no lens elements are disposed between the aperture stop and the first lens element. The photographing lens system has a total of four lens elements.