An adsorbent deterioration determining apparatus can accurately determine a deterioration of an adsorbent in a relatively simple configuration. The method includes forming a first rotatable shaft having a first longitudinal axis, the first shaft comprising an axle portion having on one end thereof an enlarged pocket end. When the beam is switched from the low beam to the high beam, pale irradiated light from the first lamp piece unit is added with pale irradiated light from the second lamp piece unit to thereby provide an impression for the driver andor passenger of unity in colors between the two lamp piece units.