The present disclosure provides an image capturing optical system comprising: a positive first lens element having a convex object-side surface; a negative second lens element having a concave object-side surface; a third lens element; a fourth lens element having a convex object-side surface and a concave image-side surface, the object-side surface and the image-side surface thereof being aspheric; a fifth lens element having a concave image-side surface concave, both of the object-side surface and the image-side surface being aspheric, at least one of the object-side surface and the image-side surface having at least one convex shape in an off-axis region thereof.