The invention provides a group III nitride semiconductor manufacturing system which is free from interruption to rotation of a rotational shaft. The group III nitride semiconductor manufacturing system has a reacting vessel having an opening, a crucible disposed in an interior of the reaction vessel and containing a melt including at least a group III metal and an alkali metal, a holding unit supporting the crucible and having a rotational shaft extending from the interior of the reaction vessel to an exterior of the reaction vessel through the opening, a rotational shaft cover covering a part of the rotational shaft positioned at the exterior of the reacting vessel and connected to the reacting vessel at the opening, a rotational driving unit disposed at an outside of the reacting vessel and regulating the rotational shaft and a supply pipe connected to the rotational shaft cover and supplying a gas including at least nitrogen into a gap between the rotational shaft and the rotational shaft cover, wherein the gas and the melt react to grow a group III nitride semiconductor crystal.