A surface position sensor provided in a projection exposure apparatus used for manufacturing, e.g., a semiconductor device so as to measure the height of the object, the surface of a wafer, in a wide measurement range with a high precision. Two beams of light (LB1, LB2) of which the frequencies are different from each other by a predetermined value and which cross at a variable angle are produced by illumination light sources (22A, 22B) and a heterodyne beam producing optical system (24). The beams crossing at variable angle are directed to the surface of a wafer (W) through a light beam projecting objective system (29A) to form interference fringes with pitches differing in a time-division manner on the surface of the wafer (W). From the two beams reflected from the surface, a heterodyne beam is generated through a sensing objective system (34A) and a received beam combining prism (37), and photoelectrically transduced to a sensing beat signal for each interference fringe of different pitch. Based on the sensing beat signal, the focal point of the surface is determined.