An optical system is provided in which flare is prevented and, when the light reflected from a reflection component is modulated by a photoelastic phenomenon, the effect of the modulation on the measurement of the intensity of the reflected light can be eliminated, and, further, a flying height tester for a magnetic head is provided using such optical system. A multilambda plate is provided in the flare-prevention optical system for allowing the light reaching a reflection component and the light reflected from the reflection component to pass, and for allowing the phase difference between the ordinary ray and the extraordinary ray to vary by 2.pi. or more depending on the wavelength of light. Since the light passed through the multilambda plate only varies in the peak position and the peak height varies little even if the light has experienced modulation, no error occurs in the spectrum intensity of the light. A depolarizer may be substituted for the multilambda plate.