Plywood used for construction as a decorative inner material is inflammable to bring large fire accidents and burn out human life and their properties. To diminish the fire disaster, fire retardant plywood has been required indeed. In the methods of manufacturing the fire retardant plywood the soaking method is occasionally used. However after soaking plywood into fire retardant chemical solutions, redrying of soaked plywood is the most important. In this study, 3.5mm thin and 5.0mm thick plywoods were selected for fire retardant treatment. Treating solutions were prepared for 20% dilute solutions of ammonium sulfate, monoammonium phosphate, diammonium phosphate, borax-boric acid and minalith, and water solution. 1-, 3-, 6-, and 9 hour-soaking treatments were applied and after treatments hot plate drying was applied to those treated plywoods at $90^{\circ}C$, $120^{\circ}C$ and $150^{\circ}C$, of press temperature. Drying rates, drying curves, water absorption rates of fire retardant chemicals, weight per volume and fire retardant degree of plywood were investigated. The results may be summarized as follows: 1. The plywoods treated with ammonium sulfate, monoammonium phosphate and diammonium phosphate and diammonium phosphate showed increase of chemical absorption rate with proportion to increase of treating time, but not in case of the plywood treated with borax-boric acid and minalith. 2. In the treatment of definite time, the absorption rate per unit of volume of plywood showed higher in thin plywood (thickness of 3.5mm) than in thick plywood (thickness of 5.0mm). In both thin and thick plywoods, the highest absorption rate was observed in 9 hour-treatment of ammonium sulfate. The value was 1.353kg/$(30cm)^3$ in thin plywood and 1.356kg/$(30cm)^3$ in thick plywood. 3. The volume per weight of plywood after chemical treatment increased remarkably and. after hot plate drying, the values were to a little extent higher than before chemical treatment. 4. The swelling rates of thickness in chemical-treated plywoods increased similarly with that of water-treated plywood in 1- and 3 hour-treatment of both thin and thick plywoods. But in 6- and 9 hour-treatment, the greater increased value showed in water-treated ply wood than any other chemical, especially in thick plywood. 5. The shrinkage rates after hot plate drying showed the same tendency as the swelling rate, and the rate showed the increasing tendency with proportion to increase of treating time in thick plywood of both chemical and water treatments. 6. Among drying curves, the curves of water-treated plywood placed more highly than chemical-treated plywood without-relation to thickness in 6- and 9 hour-treatment except in 1- and 3 hour-treatment. 7. The drying rate related to thickness of treated plywood, was twice above in thin plywood compared with thick plywood. 8. The drying rate remarkably increased with proportion to increase of the plate temperature and, the values were respectively 1.226%/min., 6.540%/min., 25.752%/min. in hot plate temperature of $90^{\circ}C$, $120^{\circ}C$, $150^{\circ}C$ in thin plywood and 0.550%/min., 2.490%/min, 8.187%/min, in hot plate temperature of $90^{\circ}C$, $120^{\circ}C$, $150^{\circ}C$ in thick plywood. 9. In the treatment at $120^{\circ}C$ of hot plate temperature, the drying rates of chemical-treated plywood showed the highest value in monoammonium phosphate of thin plywood and in diammonium phosphate of thick plywood. But the drying rate of water-treated plywood was highest in 6- and 9 hour-treatment. 10. The fire retardant degree of chemical-treated plywood was higher than that of the untreated plywood as shown in loss of weight, burning time, flame-exhausted time and carbonized area. 11. The fire-retardant effect among fire retardant chemicals were the greatest in diammonium phosphate, the next were in monoammonium phosphate and ammonium sulfate, and the weakest were in borax-boric and minalith.