The present invention achieves chemical bonding by means of a joined film made of oxides formed on a joined surface. In a vacuum container, amorphous oxide thin films are respectively formed on smooth surfaces of two substrates, and the two substrates overlap such that the amorphous oxide thin films formed on the two substrates come into contact with each other, thereby causing chemical bonding involving an atomic diffusion at a joined interface between the amorphous oxide thin films to join the two substrates.