A heat-treatment apparatus according to the present invention comprises a sheet-like heating body, which is bent like a letter "U", and a heat equalizing plate constituted by a metal plate bonded to a heat generating part of the sheet-like heating body. The heat equalizing plate is bonded to the inner surface of the sheet-like heating body bent like a letter "U". Alternatively, a fluorocarbon resin is coated on the surface of the heat equalizing plate.