An oxygen-free hydrogen plasma ashing process particularly useful for low-k dielectric materials based on hydrogenated silicon oxycarbide materials. The reference point projection unit stores reference points. The processor is further configured to determine a foldedunfolded state of the second body in accordance with the detecting signals of the first sensor and second sensor. The control section determines saturated absorption lines on the basis of an output of the differential signal, selects a specific saturated absorption line on the basis of the number of determined saturated absorption lines and a combination of the determined saturated absorption lines, and stabilizes the oscillation frequency of the laser light at the specific saturated absorption line. The accommodation plate is disposed between the first exterior member and the second exterior member.