A protection layer containing carbon as a major component is deposited by plasma CVD. The protection layer has film quality such that, when a spectrum is obtained by excluding photoluminescence from a Raman spectrum in a wavenumber band from 900 cm−1 to 1800 cm−1 obtained by exciting the protection layer with an argon ion laser beam having a wavelength of 514.5 nm and the spectrum is subjected to waveform separation by the Gaussian function to split a D peak appearing around 1350 cm−1 and a G peak appearing around 1520 cm−1, the ratio Dw/Gw between a half width Dw of the D peak and a half width Gw of the G peak exceeds 0 and is not greater than 2.7.