To provide a method for producing a nitride single crystal body such as a GaN single crystal which has high growing speed, is inexpensive, and has excellent crystallinity.

A vessel 2 made of boron nitride stores a mixture of sodium and gallium, and NaF powder in addition to Na3N powder as a nitrogen source, the inside of a pressure vessel 5 is fed with gaseous nitrogen as an atmospheric gas 6, the pressure is controlled to 4 MPa, and thereafter, a heater 1 is energized, thus the raw material in the vessel 2 is heated to 780C, so as to form a raw material melt 3. Successively, a disk-shaped seed crystal 4 made of a GaN single crystal is immersed into the raw material melt 3 whose liquid face is covered with a sealing material 7, and a GaN single crystal body 9 is grown on the seed crystal 4.