We have constructed an apparatus for in situ measurement of stress of the film prepared by sputtering using an optical non-contact displacement detector. A change of the gap distance between the detector and the substrate, caused by stress of a deposited film, was detected by a corresponding change of the reflectivity. The sensitivity of the displacement detector was 5.9 V Angstrom(-1) and thus it turned out to be good enough to detect stress caused by deposition of a monatomic layer of Co or Pd. The apparatus was applied to in situ stress measurements of Co/Pd multilayer thin films prepared on the glass substrates by d.c. magnetron sputtering. At the very beginning of the deposition, both Co and Pd sublayers exhibited tensile stresses. However, when the film was thicker than about 100 Angstrom, constant tensile stress in the Co sublayer and compressive stress in the Pd sublayer were observed, which is believed to be related to a 9% lattice mismatch between the matching planes of Co and Pd.