Abstract: A modular furnace, in particular for the oxidative stabilization of a carbon fiber starting material comprising a cuboidal furnace chamber, on the upper face of which first deflecting rollers are arranged in a mutually spaced and parallel manner and on the lower face of which second deflecting rollers are arranged in a mutually spaced and parallel manner such that the carbon fiber starting material runs upwards and downwards in a laterally adjacent and slightly spaced manner so as to meander vertically in the area of the furnace chamber. A carbon fiber inlet locking device and a carbon fiber outlet locking device are provided on the upper face of the furnace chamber, and an air guiding device is connected to the furnace chamber. A supply air portion of the air guiding device is connected to a vertical air inlet side of the furnace chamber, and a discharge air portion of the air guiding device is fluidically connected to a furnace chamber vertical air outlet side opposite the vertical air inlet side.