To provide a controller with gas blending arithmetic operation that can be interlocked easily with temperature control and can control the flow rate and components and composition of an atmospheric gas.

The controller 1 of a furnace having flow rate control means which individually control the flow rates of a plurality of component gases gi forming the atmospheric gas calculates the required flow rates Fi of the gases gi based on the set flow rate value Fh of the atmospheric gas and the set concentration value Csp(n) of a specific component gas gn in the atmospheric gas and gives the calculated flow rates Fi to the flow rate control means. Preferably, a means which stores the secular changing patterns of the set flow rate value Fh of the atmospheric gas and the set concentration value Csp(n) of the specific component gas gn is provided to calculate the required flow rates Fi of the component gases gi in a prescribed time period based on the set flow rate value Fh and changing patterns of the values with time Fh and Csp(n). It is possible to collectively control the furnace temperature and atmospheric gas of the furnace in an interlocking way by incorporating a function which controls the furnace temperature by inputting the secular changing pattern of temperature into the controller 1.