For the system of polycrystalline Si/SiO2(thermal)/Si(001), the strain field introduced into the Si substrate was studied with the asymmetric X-ray diffraction method. Polycrystalline Si layers with a thickness of 10.0 nm were grown on 4.5-nm thick thermally grown oxide layers. To study the strain fields, the rocking curves for Si {113} planes of the bulk films were measured. The intensity of the rocking curves increased for polycrystalline Si-grown films, whereas the intensity must be decreased by the absorption of X-rays in the polycrystalline Si layer. This indicates that the strain is introduced into the substrate. The Darwin dynamical calculation for a distorted crystal indicates that the intensity enhancement of a rocking curve can occur as a compressive strain. From a comparison of the measured and calculated curves, we conclude that the compression of the {001} spacing near the substrate surface occurs during the growth of the polycrystalline Si layer.