PURPOSE: To heighten joining characteristics and joinability by cleaning the joining faces by applying high frequency voltage to the joining faces.

CONSTITUTION: Joining faces of joined materials 11, 12 are opposed to each other providing an arbitrary distance 4. A connecting conductor 6 is connected to a part other than the joining faces of joined materials 11, 12 and joining allowance 5, and a high frequency high voltage power source is connected to impress high frequency high voltage to the joining faces of joined materials 11, 12. Oxide films existing on the joining faces of joined materials 11, 12 are removed and very clean metal surfaces are exposed. Then, cleaned metal surfaces are brought into contact, and pressed in the direction of the arrow A by a pressing force supplying device. The joined faces are unified by giving plastic deformation and joined.