A device and method for bonding objects to be bonded each having a metal bonding portion on a substrate, comprising cleaning means for exposing the metal bonding portions to a plasma having an energy enough to etch the surfaces of the metal bonding portions at a depth of 1.6 nm or more over the entire surfaces of the metal bonding portions under a reduced pressure and bonding means for bonding the metal bonding portions of the objects taken out of the cleaning means in an atmospheric air. By using a specific scheme, metal bonding portions after the plasma cleaning can be bonded in the atmospheric air, thereby significantly simplifying the bonding process and the whole device and lowering the cost.