For the purpose of an amelioration of preservative impregnability, three types of pretreatment: the steaming, the explosion and the boiling, were tested. The specimens were prepared by the domestic four species: Itajii, Ryukyumatsu, Sugi and Hinoki, and 1 refractry imported species: Douglas-fir. The dimension of specimens was 20 x 20 x 300 mm³ and were treated with CCA in a laboratory OPM machine. Three OPM schedules and Bethel process without after vacuume were tested in this experiment. The results obtained were follows: 1. In the case of the treatment of green woods, the calculated retentions of preservatives based on those initial mass, were periodically slowly increased or sometimes decreased. This reason was considered the replacement of the free water in wood to the preservative solution. The sapwood of rather easy treatable species like Ryukyumatsu, was obtained good penetration by Bethel process. The sapwoods of treatable species like Sugi and Hinoki, were obtained best penetration by OPM-1 or by OPM-2. The combination of the steaming and the OPM was rather good results for all species tested of green woods. 2. In the case of the treatment of the air-dried woods, the combination of the steaming and the OPM was obtained rather good results for all species tested included the refractry specimens like Douglas-fir.

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