A photographing optical system includes five lens elements which are, 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 and a fifth lens element. Each of the five lens elements has an object-side surface facing toward the object side and an image-side surface facing toward the image side. The second lens element has negative refractive power. The fifth lens element has negative refractive power. At least one of the five lens elements has at least one aspheric surface having at least one inflection point.