PURPOSE: To form a firm thick hardened layer on an object to be treated immersed in a fluidized bed furnace by forcibly circulating gaseous NH3 to be supplied into the fluidized bed furnace used as a gas nitriding furnace.

CONSTITUTION: An object 5 to be treated is placed into a fluidized bed furnace 1 loaded with fluid powder 4 of alumina or the like, and after the furnace is tightly closed by being put with a cover 6, gaseous N2 is supplied 8 to substitute the air in the furnace 1 with the gaseous N2. Next, a blower 7 is driven to circulate the gaseous N2 in an arrow 14 direction, and after the inside of the furnace 1 is heated to a prescribed temp. by a heater 13, gaseous NH3 is supplied 8 and is fed and circulated into the furnace 1. The fluid powder 4 in the furnace 1 is fluidized and the object 5 is nitrided. During this nitriding, the concn. of the gaseous NH3 in the circulating gas is measured and when the concn. lowers below a prescribed value, the gaseous NH3 is supplied 8 and part of the gas is exhausted 11 to maintain the decomposition rate of the gaseous NH3 within a prescribed range. For example, a steel is used for the object 5, and a thick and firm nitrided layer of less variance is formed on the surface thereof.