A process for producing an aluminum-containing III nitride single crystal, comprising: a reaction step of bringing a halogen gas and aluminum into contact with each other at a temperature of 300 to 700°C inclusive, thereby producing a mixed gas comprising a gas of an aluminum trihalide and a gas of an aluminum monohalide; a conversion step of setting the temperature of the mixed gas at a temperature that is equal to or higher than a temperature at which a solid of the aluminum trihalide is precipitated and that is lower by 50°C or more than a temperature at which the halogen gas and aluminum have been reacted with each other in the reaction step, thereby converting the aluminum monohalide in the mixed gas into a solid substance; a separation step of separating the solid substance from the gas, thereby removing the gas of the aluminum trihalide; and a crystal growth step of using the gas of the aluminum trihalide as an aluminum nitride single crystal raw material without lowering the temperature and while keeping the temperature at a temperature that is equal to or higher than the temperature employed in the conversion step.