ment, such a structure is employed that, in place of the formation of the light shielding member 27 in the first embodiment, a light shielding portion 27B (light shielding portion) is formed integrally on the peripheral portions of transmit windows 2R, 2L to project from the inner side surface of the case 3, and then areas other than the effective area out of the surface of the prism 4 are covered