A highly reliable copper circuit-joined board that, in mounting a semiconductor element, a lead frame or the like on a ceramic substrate, enables the semiconductor element, the lead frame or the like to be strongly joined to the substrate without breaking or deformation of the substrate found in conventional joining methods, such as brazing and joining using a copper/copper oxide eutectic crystal. Any one of an interposing layer comprising a brazing material layer comprising silver and/or copper as a main component and an active metal or an interposing layer having a two-layer structure comprising a first interposing layer comprising the brazing material layer or a high-melting metallizing layer and a second interposing layer, having a melting point of 1000.degree. C. or below, comprising Ni, Fe, Cu as a main component in that order from the substrate side, is formed on a ceramic substrate, and a conductor layer, comprising copper as a main component, which, in both the lengthwise and widthwise directions, is at least 0.05 mm shorter than the interposing layer, is formed on the interposing layer to prepare a copper circuit-joined board. The copper circuit-joined board may comprise the base board having thereon an outer layer comprising Ni as a main component. A semiconductor element is mounted on the copper circuit-joined board to prepare a semiconductor device.