The present invention in the volume fraction of the control, the final heat treatment step of the ferrite (ferrite), Perlite (pearlite) for establishing the optimum heat treatment conditions and the optimal process of the gears (gear) steel and machinability improvement through the development of isothermal annealing process technology deformation control of the heat treatment, for uniformity of band structures and the like (band structure), bainite (bainite) in the mechanical properties, to a method of heat-treating a steel type developed isothermal annealing for the purpose of removal of perlite (pearlite) bunching. The present invention to a primary heating to heat to maintain the temperature of 930 ~ 950 ℃ ℃ added after heating 120 ~ 150 minutes thermal treatment material And it flows into the first annealing in the primary heating process at the same time and are continuously interlocked ends, Maintaining the temperature of 10 ℃ to pipes that are installed on the outer surface ~ 20 ℃ and round-trip 80 ℃ per minute, the internal temperature of the internal temperature by the cooling water that is circulated to the first annealing in a state maintaining the 600 ℃ ~ 630 ℃ ~ 100 ℃ was rapidly cooled to the internal temperature is cooled by gives 5 ~ 8 minutes at a first annealing being kept in a 600 ℃ ~ 630 ℃, Again, the internal temperature in the state in which the inlet to the second annealing maintained at 650 ~ 680 ℃ ℃ gives to 5-8 bungan maintaining low temperature, When the five or working time of 8 minutes for the low temperature maintained in the in the second annealing lapse heat treated material was again introduced in to the last tempering was kept the inner temperature to 680 ℃ ~ 700 ℃ maintain a temperature of 120 ~ 210 bungan is one characterized in that the heat treatment.