A cam face 303a of a cam 303 has a shape depending on the distribution of the light amount in the cross-section of reference light LR. When the cam 303 is rotated by a stepping motor 302, an abutment 312 moves following the displacement of the cam face 302a with the rotation of the cam 303. A light-blocking link 310 rotates about the axis of rotation 311 with the movement of the abutment 312. A shield 313 moves in a first direction with the rotation of the light-blocking link 310 to change the shielding region of the reference light LR. A light-blocking plate 400 can shield the reference light LR from a second direction different from a shielding direction (first direction) by an attenuator 300. The light-blocking plate 400 is moved by a drive mechanism 410 and changes the shielding region.