An object is to provide a composite board of plaster and inorganic fiber excellent in fire resistance, provided with sound-absorbing property, which is absent from a plasterboard, and heat insulating property, light in weight, and high in dimensional stability to humidity change. The composite board of plaster and inorganic fiber is provided, which is characterized in that the proportion of the inorganic fiber ranges 20 to 70% by weight with respect to the total weight of the composite board. Since the composite board of the present invention contains inorganic fiber, it becomes a lightweight plasterboard and excellent in sound absorbing property. In addition, since the composite board of the present invention uses plaster, which is excellent in dimensional stability, as a binder, it is excellent in dimensional stability to humidity change compared to a rock wool decorative sound-absorbing board using a large amount of starch as a binder.

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