Abstract: Bodies of at least one material are held in a contacting holder 12 in a vacuum chamber. The surfaces of the bodies are cleaned by a low energy ion etching. Water vapor from a pure water bottle is supplied through a nozzle as a water molecule beam so that water molecules and hydroxide groups are chemically adsorbed on the surfaces of the bodies. A plasma beam or microwaves are applied to the surfaces of the bodies to remove the water molecules and leave only hydroxide groups remaining on the surfaces. The holder is operated to bring the surfaces of the bodies into contact with each other, to thereby obtain direct bonding of the bodies.