As a result of tests on the administration of additives in the production of the shiitake mushroom (Lentinus edodes (Berk.) Sing.), the following was made clear. 1. Antagonism of Shiitake Fungi with Hupocrea spp. Fungi An antagonistic phenomenon was discovered between shiitake fungi and Hypocrea spp., an injurious fungi. Namely, results showed if shiitake fungi did not completely cover the surface of a medium, such as an agar medium, Hypocrea spp, invaded the shiitake fungi. On the other hand, if the surface of the medium was covered throughout, the shiitake fungi showed sufficient resistance against Hypocrea, and shiitake fungi increased its biomass. Further, other important factors such as; culture temperature, the water content of a medium, pH, the amount of spores of Hypocrea spp. fungi or the like were also found. Accordingly, as a measure to protect shiitake fungi from injurious fungi, it is extremely important to grow shiitake fungi rapidly to keep a high mycelial biomass in the medium. Results indicate that initial activity of shiitake fungi is an important factor in protecting from an injurious fungi (Chapter I). 2. Effect of Additives in Indoor Tests As a means for promoting the growth of shiitake mycelium, several additives were tested, that is; hot water extracts of natural substances centering around vegetables; liquor-components from sulfite pulping (lignin carbohydrate complex sulfonate) ; hot water extract of chlorella; or the like. They were chosen to perform experiments in a laboratory, at first. The liquor components from sulfite pulping and chlorella extract were effective in the agar medium. Chlorella and welsh onion extracts were effective in the wood flour medium. Lastly, carrot and orange peel extracts were effective with the small log. Following tests for selecting proper additives, a sawdust substrate cultivation test was conducted. To promote the growth of mycelium, the administration of the welsh onion extract was applied as an additive, resulting in a shorter spawning period. Furthermore, successive growth of mycelium was positively affected in succeeding generations, resulting in an increase in the production of fruit bodies (Chapter II). 3. Determination of Mycelial Biomass An idea was pondered as to whether the administration of additives would promote the production of shiitake mushroom on natural bed log cultivations. There was no proper means, however, for accurately determining the mycelial biomass of shiitake fungi in a bed log. Therefore, it was difficult to judge the effect of additives discovered in indoor tests. In the production of shiitake mushroom, though the accurate determination of the mycelial biomass is an essential matter for the formation of a fruit body, a weighing method has not been established up to date. Therefore, the establishment of a method for judging the mycelial biomass was considered to be a necessary matter in order to perform an investigation on the administration of additives. Some methods were effective, that is; measuring color reaction; the evolution quantity of carbon dioxide; and, the surface whiteness of a medium. Especially, it was shown that the mycelial biomass in bed log could be simply and accurately determined by color reaction generated when bromophenol blue, a pH indicator, was sprayed (Chapter Ⅲ). 4. Effect of Additives in Field Tests The amount of fruit bodies harvested from natural bed logs was analyzed for five years. The effect on the administration of additives was confirmed in field tests. In bed logs having greater mycelial biomass, by means of the administration of additives, the spawn run time was shortened. As a result, the damage rate caused by Hypocrea spp. decreased and, further, in succeeding generations, the amount of fruit bodies was increased. This effect was cofirmed in almost all seven species of trees, that is; Quercus mongolica ; Q. serrata ; Q. acutissima ; Q. glauca ; Castanopsis cuspidata ; Pasania edulis ; and Persea thun