To provide a method of atmosphere control in a heat-treating furnace, whereby desirable heat-treatment can be achieved through effective inhibition of excessive overshoot of a carbon potential index value.

A controller controls the index value of carbon potential to a prescribed value according to the furnace temperature by regulating the degree of opening of a flow control valve which feeds endothermic gas to the furnace. Here, the carbon potential is defined as a ratio of the CO2 gas concentration to a square value of the CO gas concentration in the furnace atmosphere. Within the range where a target index value remains constant, as soon as a measured index value falls within a prescribed range relative to the constant target index value, the controller reduces the degree of opening of the valve at a predefined reduction rate, which can be zero, according to the current degree of opening.