To perform heat treatments having different treating targets in one continuous heat treatment furnace for metallic strip composed of a heating zone, a soaking zone, a slow cooling zone and a cooling zone by eliminating shifting of atmosphere in the furnace from the slow cooling zone to the cooling zone.

In the continuous heat treatment furnace T for metallic strip constituted of the heating zone 1, the soaking zone 2, the slow cooling zone 3 and the cooling zone 4, a first cooling atmospheric circulating path Pa supplying the atmosphere in the furnace taken out from the cooling zone with a fan 5 into the cooling zone through a flow rate adjusting valve V4 at the cooling zone after cooling is arranged in a cooler 6. Further, this furnace is constituted by arranging a second cooling atmospheric circulating path Pb selectively supplying cleaned atmosphere which is obtained by cooling the atmosphere in the furnace taken out from the slow cooling zone with a fan 7 by using a cooler 8 and removing the oil content in the atmosphere in the furnace condensed by the cooling by using an oil content removing device 9, to the heating zone, the soaking zone and the slow cooling zone respectively through flow rate regulating valves V1, V2, V3.