To improve safety against lack of oxygen in working with a continuous tunnel furnace.
When a surface active agent included in an electrode catalyst layer 20 for a fuel cell is thermally decomposed and removed by the continuous tunnel furnace 1, an oximeter 10 is used so that a nitrogen gas is supplied from a nitrogen gas supply opening 7 to a high-temperature part near a center in a state of keeping oxygen concentration not causing firing or the like of catalytic metal. The nitrogen gas flows to low-temperature parts 5a, 5b near an inlet/outlet opening with a decomposed gas of the surface active agent. The oxygen concentrations of the low-temperature parts 5a, 5b are measured by the oximeters 10a, 10b, air is supplied from air supply openings 8a, 8b to adjust the oxygen concentration of the exhausted gas to be 20% or more, and the gas after the adjustment is exhausted from exhaust openings 9a, 9b. By adjusting the oxygen concentration of the gas in the furnace to be high before exhausting, lack of oxygen can be prevented, and the heat treatment work can be performed in safety.