A conceptual design for a heliotron type nuclear fusion reactor is studied. Making most use of the advantageous features of heliotron, the reactor core is designed to be as compact as possible. The superconducting helical coil and cryostat are continuously wound around the vacuum vessel. Disassembly, assembly and repair of core components such as divertor, first wall and blanket modules are accomplished through port spaces between the slots of the helical coil. The reactor core is designed as compactly as possible. The large aspect ratio simplifies the maintenance work.