A furnace for the formation of a black oxide film on the surface of a thin metal sheet is disclosed which comprises a tunnel-like furnace proper provided at one terminal side thereof with an inlet and at the other terminal side thereof with an outlet, conveying means laid inside the furnace proper from the inlet through the outlet thereof for conveying a thin metal sheet from the inlet to the outlet, openable shutter means for partitioning the interior of the furnace proper into at least first and second regions on the front and rear sides respectively in the direction of conveyance of the thin metal sheet, first gas supply means for feeding into the first region on the inlet side of the furnace proper partitioned by the shutter means a mixed gas containing carbon dioxide and carbon monoxide and containing substantially no oxygen or a mixed gas containing carbon dioxide, carbon monoxide, and steam and containing substantially no oxygen, second gas supply means for feeding into the second region on the outlet side of the furnace proper partitioned by the shutter means a mixed gas containing carbon dioxide, carbon monoxide, and oxygen and containing substantially no steam, and heating means for heating the first region to a temperature in the range of 500.degree. to 650.degree. C. and the second region to a temperature in the range of 100.degree. to 300.degree. C.