The behaviour of slightly non-ideal base-gas in the magnetic field is first studied; it is shown that such a system can be considered as a model of a superconductor of the second kind. And explanation of abnormal temperature dependence of the magnetic field penetration depth in high-temperature metal-oxide superconductors is first givenAvailable from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio