There are provided an intermetallic compound superconductor that is high in superconducting transition temperature, and an alloy superconductor that is high in superconducting transition temperature and excels in malleability and ductility, as well as a method of making such a superconductor with good reproducibility and at a low cost of manufacture. This entirely new intermetallic compound superconductor is made of magnesium (Mg) and boron (B) and has a chemical composition expressed by formula: Mg₁B₂, has a hexagonal AlB₂ type crystallographic structure and has a superconducting transition temperature (T_c) of 39 K. An alloy containing this intermetallic compound excels in malleability and ductility and constitutes the alloy superconductor having a superconducting transition temperature (T_c) of 39 K. In the method of manufacture, a Mg containing feedstock powder and a B containing feedstock powder are mixed together to form a mixture thereof which is, e.g., hot pressed to produce a semiconductor product.