The lattice parameter of Ba0.6 K0.4 BiO3, which is a well-studied conventional superconductor, was measured by the X-ray Rietveld method between 10 K and room temperature. A very small change in the lattice parameter could be detected in the superconducting phase. The change could be attributed to a spontaneous strain generated in the superconducting phase by the coupling between a superconducting order parameter and the strain. Previously published data on Y Ba2 Cu3 O6.5 and MgB2 were analyzed, and the present results were compared with our previously obtained results on La1.85 Sr0.15 CuO4. It was concluded that this coupling phenomenon is common to all superconductors.