A method of producing an n-type group III nitride single crystal includes putting raw materials that include at least a substance including a group III element, an alkali metal, and boron oxide into a reaction vessel; melting the boron oxide by heating the reaction vessel to a melting point of the boron oxide; forming a mixed melt which includes the group III element, the alkali metal, and the boron oxide, in the reaction vessel by heating the reaction vessel to a crystal growth temperature of a group III nitride; dissolving nitrogen into the mixed melt by bringing a nitrogen-containing gas into contact with the mixed melt; and growing an n-type group III nitride single crystal, which is doped with oxygen as a donor, from the group III element, the nitrogen, and oxygen in the boron oxide that are dissolved in the mixed melt.