To provide a method for finely finishing a fiber from straw which can be used for a fiber board, and a method for producing a cellulosic material such as a fiber board.

Straw is cut to the length range of about 2-4", then after cleaning, is softened under a pressurized vapor treatment, and is finely finished using a pressurized mechanical refiner 26. Further, the straw is passed through a Fourdrinier machine 30, and is sent to a drying zone 32 to produce a fiber which can be used for the manufacture of a cellulosic fiber board product. This straw fiber can be mixed with any other different type of fiber at a freely chosen ratio, and can be used for the manufacture of a soft fiber board product, an intermediate density fiber board product and a hard fiber board product.