A coil (11) for use in growing a single crystal by the floating zone method is formed as an annular single-turn coil having a wedge-like configuration in vertical section, such that it progressively increases in thickness from the inner circumference to the outer circumference of the coil. The surface of the annular body of the coil positioned on the polycrystalline side of the semiconductor rod (12) slopes radially upward, at an angle to a plane perpendicular to the crystal growth axis. The coil has an annular thin projection (11a) which projects upwardly from the coil on the polycrystalline side, at or near its inner circumference. The projection serves to increase the magnetic flux density around its tip portion, thereby facilitating the molten state of the polycrystalline portion near the tip.
