At least either of the light entering plane or the light exiting plane is parallel to the (111) crystal face of the CaF2 crystal and the laser beam entering from the entering plane passes through the plane located between the [111] axis and the first azimuth axis in the locus of rotation of the [001] axis around the [111] axis and including the [111] axis and the first azimuth axis, the plane located between the [111] axis and the second azimuth axis in the locus of rotation of the [010] axis around the [111] axis and including the [111] axis and the second azimuth axis or the plane located between the [111] axis and the third azimuth axis in the locus of rotation of the [100] axis around the [111] axis and including the [111] axis and the third azimuth axis and exits from the exiting plane.