A reflow furnace using a conventional heater for blowing hot air has difficulty in reducing Δt and in stabilizing the oxygen concentration at a low level. In addition, it is difficult to uniformly discharge hot air from the discharge holes in a perforated plate of a conventional heater for blowing hot air. In a reflow furnace according to the present invention, the total area per unit area of discharge holes formed in a perforated plate in a heater for blowing hot air installed in a main heating zone is 1.5-5 times the total area per unit area of the discharge holes formed in a perforated plate of a heater for blowing hot air installed in a preheating zone. A heater for blowing hot air has a body divided into three chambers by partitions.