PURPOSE: To reduce or eliminate an adverse influence upon an image even if an air bubble is mixed with cooling water by making the thickness of the center part of a cooling water passage in the width direction less than the thickness of a side part.

CONSTITUTION: The thickness of the center part of the cooling water passage 2 is reduced, so the air bubble becomes hard to pass through the center part of the cooling water passage 2. Specially, a large air bubble which affects the image can not pass through the center part of the cooling water passage. Consequently, the air bubble passes through the side part of the cooling water passage which is thick and does not passes through an irradiation range C of backlighting. Consequently, the adverse influence of the air bubble upon the image is reduced or eliminated.