To uniformly heat whole area of a comparatively long convection heating area having full length of 100 meters or more by one hot air supply system.

In a drying furnace having the convection heating area A2 in which hot air is blown for a workpiece W conveyed in a furnace main body 3 to dry it, auxiliary blowers 6F, 6R for accelerating heating air in a hot air blow-out duct 5 toward duct end parts 5F, 5R are arranged between a hot air flow-in port 5in of the hot air blow-out duct 5 arranged over the whole length from a carry-in side end part in the convection heating area A2 to a carry-out side end part and the duct end parts 5F, 5R.