In a processing section 4, allowable gain-value range defining unit 41 measures a gain value k corresponding to a level ratio between an MPP signal and an SPP signal generated by a tracking error signal generating circuit 3 to define an allowable range of the gain value, and eccentricity measuring unit 42 measures an eccentricity of an optical disk. If the eccentricity is determined to be greater than or equal to a predetermined value by eccentricity determining unit 43, gain-value defining unit 44 selects a minimum value of the gain value k to define the minimum value as a gain value k of an amplifier 37. On the other hand, if the eccentricity is determined to be less than the predetermined value, the gain-value defining unit 44 selects a central value of the gain value k to define the central value as the gain value k of the amplifier 37.