To inexpensively provide a fire-resistant incombustible highly adiathermic plate material having fire resistance, incombustibility, lightness in weight, and besides extremely high adiathermacy.

The fire-resistant incombustible highly adiathermic plate material is configured so that a highly adiathermic layer is formed between two inorganic plate materials having a predetermined thickness and area, and fire resistance and incombustibility, the highly adiathermic layer obtained by applying an adiathermic coating material, prepared by compounding pearlite fine particles having an average particle size of not greater than 2 mm and an apparent specific gravity of not greater than 0.2, 10-25 vol.% by a ratio of a siloxane and silanol salt high-molecular weight solution, in a predetermined thickness, and heat-drying to mutually adhere and solidify the pearlite fine particles strongly, and to integrally adhere onto the inorganic plate materials.