PROBLEM TO BE SOLVED: To provide a metal composite body that allows improving junction reliability when a metallic member and another member are bonded via a junction layer composed of a calcined body containing at least one of Ag particles and silver oxide particles or both and an organic material, a circuit board composed of the metal composite body, a semiconductor device including the circuit board, and to provide a method of manufacturing the metal composite body.SOLUTION: A metal composite body 10 comprises: an insulating layer 11; and a metallic member 12 disposed on one surface or the other surface of the insulating layer 11. An Ag base layer 12a for Ag sinter bonding is formed on the surface opposite to the surface of the metallic member 12 on which the insulating layer 11 is disposed, and the metallic member 12 and the Ag base layer 12a are solid-phase diffusion bonded.
