The invention provides a compact multistage furnace of which the installation area in a factory is decreased. A multistage furnace is configured by piling up a plurality of furnace units in the vertical direction. Each of the furnace units includes an upper heater and a lower heater layered in the vertical direction and holding a heat insulator therebetween, a support pipe disposed on one end of the upper heater and extending in the horizontal direction, a support pipe disposed on other end of the upper heater and extending in the horizontal direction, and a plurality of work support bars mounted over the support pipes. The back surface of a work supported by the work support bars is opposed to the upper heater and the front surface of the work is opposed to the lower heater of the adjacent furnace unit disposed above.