<p>A molding formed therein with a pathway in which liquid flows having its upper surface which is planer, and made of a transparent material at least in an upper part above the upper surface of the pathway, is provided, a light beam is projected to the molding at an incident angle in a range from 42 to 62 deg. with respect to the normal to the upper surface of the pathway, and an intensity of a light beam reflected upon the upper surface of the pathway or transmitted through the pathway is measured in order to surely detect the presence of the liquid in the pathway. <IMAGE></p>
