There is disclosed an optical data storage medium in which the weather resistance of the semitransparent reflective film is improved and the adhesive property between the semitransparent reflective film and a substrate is enhanced, and which has a higher reliability. In an optical data storage medium (6), on the side on which a reproduction light (8) is incident, a first information layer (9) is located while on the side opposite to the side on which the reproduction light (8) is incident, a second information layer (10) is located. A semitransparent reflective film (3) of first information layer (9) is AgPdCu alloy thin films containing 0.5 to 3.0 weight % Pd and 0.1 to 3.0 weight % Cu or AgPdTi alloy thin films containing 0.5 to 3.0 weight % Pd and 0.1 to 3.0 weight % Ti. At the wavelength 650 nm, the optimum film thickness of AgPdCu alloy thin film is 5 to 18 nm and the optimum film thickness of AgPdTi alloy thin film is 10 to 25 nm. At the wavelength 450 nm, the optimum film thickness of AgPdCu alloy thin film is 10 to 25 nm, and the optimum film thickness of AgPdTi alloy thin film is 15 to 25 nm.