A secondary battery includes a plurality of battery cells and measures voltages of the battery cells. The arc tube includes an auxiliary mirror that reflects the light radiated from the arc tube toward the region being illuminated into a direction of the ellipsoidal reflector. The column pipe is inserted slidably in an insertion hole of a pivot housing that is fitted to be able to swing freely and be fixed to the front of the column housing in the axial direction. A headrest frame is provided with an engagement disk which is changed over between a lock position where the engagement disk engages with the lock plate to disable the headrest to rotate about the horizontal section, and an unlock position where the engagement disk disengages from the lock plate to enable the headrest to rotate around the horizontal section.