An image picking-up system includes seven lens elements, which are, in order from an object side to an image side, a first lens group including a first lens element and a second lens element, a second lens group including a third lens element and a fourth lens element, and a third lens group including a fifth lens element, a sixth lens element and a seventh lens element. The third lens element has positive refractive power. The fourth lens element has an image-side surface being concave in a paraxial region thereof. At least one surface of object-side surfaces and image-side surfaces of the seven lens elements includes at least one inflection point.