PURPOSE: To obtain a half mirror with high reflectance, low absorptance, flat spectral characteristics and high endurance by forming a higher refractive index thin film and an Al thin film on a transparent substrate in order in a predetermined thickness each or further forming a lower refractive index thin film and a higher refractive index thin film on the Al thin film in order in a predetermined thickness each.

CONSTITUTION: ZrO2 vapor deposited layer H of λ/4 thickness of 550mμ wavelength is formed on glass substrate 18, and Al vapor deposited layer Al of about 120 thickness on layer H. ZrO2 thin film H of λ/4 thickness, Al thin film Al of about 75 thickness, MgF2 thin film L of λ/4 thickness and H of the same thickness may be formed on substrate 18. The resulting half mirror has high reflectance R, low absorptance A, flat spectral characteristics and high endurance.