There is provided a novel method for producing a nitride single crystal with both a rapid crystal growth rate and high crystal quality, as well as a novel autoclave that can be used in the method. The invention provides a method for producing a Ga-containing nitride single crystal by an ammonothermal method, comprising introducing at least a starting material, an acidic mineralizer and ammonia into an autoclave, and then growing a Ga-containing nitride single crystal under conditions wherein the temperature (T1) at the single crystal growth site is 600° C. to 850° C., the temperature (T1) at the single crystal growth site and the temperature (T2) at the starting material feeder site are in the relationship T1>T2, and the pressure in the autoclave is 40 MPa to 250 MPa, as well as an autoclave that can be used in the method.