For bonding pure beryllium to a copper alloy, a beryllium-copper material comprising a single layer or multiple layers having a thickness of 0.3-3.0 mm and containing at least 50 atomic % of Cu is inserted between the pure beryllium and the copper alloy to prevent bonding strength from degrading in the bonding process or during operation of a nuclear fusion reactor, by effectively mitigating formation of brittle intermetallic compounds and generation of thermal stress at the bonding interface.