A cavity with a volume of 1 to 10(-3) mu m(3) is found to form at the surface of a silicon single crystal covered by a Si3N4/SiO2 film during thermal treatment in N-2. The cavity can be seen only in the central area of the film-covered region, not near the film edge, whenever the thickness of the Si3N4 film exceeds 350 nm. The volume of the cavity strongly depends on the thickness of the Si3N4 film and increases with the length of the thermal treatment. Cavity formation in wet O-2 is similar to that in N-2 except for the temperature dependence of the cavity growth. From the experimental results, it can be inferred that a migration of silicon atoms under Si3N4/SiO2 film-induced stress results in the cavity formation. It is also inferred that a depression observed in the film-covered region after Secco etching is due to Si3N4/SiO2 film-induced stress.