1. A method of detecting optically tire information marks formed on a surface of side wall of tire comprising the steps of; illuminating the side wall of tire from a first direction substantially perpendicular to the surface of side wall of tire and from a second direction inclined with respect to the surface of side wall of tire; picking-up an image of the surface of side wall of tire by means of an image pick-up device to produce an image signal; and processing the image signal supplied from the image pick-up device to detect the tire information marks.