The apparatus for manufacturing the fiberboard is in the form of two vertically opposed endless conveyors upon each of which a fiber layer is formed. Each conveyor has a slanted running portion as a part thereof. A suction box is disposed along the inner surface of each of the slanted running portions so that a fiber layer producing section is formed in each slanted portion. A passage is formed between the conveyors to direct a slurry flow of fibers along each conveyor. The relative speeds of the slurry flow through the passage along the surface of the conveyors is adjusted to be different than the running speed of the conveyors; a supply duct for the slurry flow is disposed in the center of said passage and has an outlet near the upper ends of the fiber layer forming sections. The two layers simultaneously formed can be placed upon each other to form a single layer of fiberboard.