To provide a method for stably heat-treating a thin sheet having good surface smoothness without causing troubles such as breaking using an oxidized fiber sheet as a raw material for heat treatment in a heat-treating furnace equipped with heating rollers in the furnace.

A polyacrylonitrile oxidized fiber sheet 2 is carried in the heat-treating furnace 6 equipped with a graphite roller 10 and a brushing means 12 composed of a carbon fiber material, brought into slide contact with the graphite roller surface and brushing the graphite roller surface in the furnace. The oxidized fiber sheet 2 running in contact with the graphite roller surface is heated at 500-2,500°C.