Provided is an optical disc apparatus including an interference-type optical disk signal detection system, which allows an optical path difference between two kinds of light to be controlled easily, which has a high signal amplification effect, and which is suitable for miniaturization of an optical system. An interference-type optical system is integrally formed to stabilize an optical path difference between signal light and reference light. At the same time, means for controlling an optical path length of the reference light is attached to a reference mirror, and thereby controls signal amplitude so as to be constantly set the maximum in response to a change in the optical path length with time due to a variation in the thickness of a cover layer of an optical disk, a temperature and the like. Moreover, the optical system is miniaturized by using a Twyman-Green type interference optical system.