Retarder and optical head device installing the same

An adhesive is coated on at least one surface of a thin film of organic material having birefringent properties and a phase-difference producing function, and a fixing substrate having transmitting or reflecting properties is bonded to the thin film of organic material by the adhesive, wherein materials satisfying relations of E₁2 and E₃2 where E₁ is the linear expansion coefficient of the thin film of organic material, E₂ is the linear expansion coefficient of the adhesive and E₃ is the linear expansion coefficient of the fixing substrate, and the glass transition temperature of the thin film of organic material being 150 or more, are selected. With such structure, temperature characteristics of a phase difference can be obtained so as to reduce a temperature change of an oscillated wavelength emitted from a semiconductor laser source whereby the phase difference can be kept constant even when there is a fluctuation in a wavelength of light emitted from the semiconductor laser source due to temperature.