To provide a method and an apparatus which do not need an expensive measuring instrument and are further capable of easily measuring eccentricity of any disk.

According to the method of this invention for measuring eccentricity of an information recording disk, the eccentricity of the disk is measured by irradiating a light beam while moving it so that it crosses the boundary part of a mirror part and a format area on the disk, and detecting variation in quantity of the return light. The variation in quantity of return light due to the difference in the irradiation surface form between the mirror part and the format part of the disk is obtained for each angular coordinate when the disk is rotating. The eccentricity of the disk can be obtained by obtaining a moving amount of the light beam for each rotational coordinate when the above return light has reached a predetermined quantity of light, and obtaining the difference between the maximum moving amount and the minimum moving amount.