The button mushroom, Agaricus bisporus, is one of the major economical crops cultivated in Korea. This mushroom showed the 5th production to 10,996 M/T in 2012. Several bacteria are known as the causal agents of diseases of the cultivated button mushroom (Agaricus bisporus) and oyster mushroom (Pleurotus ostreatus). Pseudomonas agarici is the causal agent of browning disease of commercial mushrooms. Colonization of mushroom caps by the bacterium results in development of browning lesions on pileus. These lesions are superficial brown spots and can be round or spreading. But P. agarici never caused sunken lesions and rotting of the mushroom tissues. A Gram-positive bacterium was isolated from mushroom media that markedly showed the antagonistic activity against Pseudomonas agarici, the most destructive pathogen of cultivated mushrooms. The HC42 strain was selected as antagonistic bacterium by inhibition zone method and it was identified as Bacillus safensis. by the cultural, morphological and physiological characteristics, and analysis of the 16S rDNA.. The isolated bacterium is saprophytic but not parasitic nor pathogenic to cultivation mushroom. The isolated bacterium for P. agarici cell, was sufficient for inhibition in vitro. Inoculation of the isolated bacterium prevents the development of bacterial disease in Agaricus bisporus. Control efficacy of browning disease of strain HC42 treatment was 66% on Agaricus bisporus. The optimal culture medium for the antagonistic bacteria growth was determined as follows: 1.5% D-galactose, 1.5% yest extract, 1% NH4Cl, 1.5% KCl, and 1.0% L-asparagin at pH 6.0 at 25℃. The suppressive bacterium may be useful in future for the development of biocontrol system and the construction of genetically modified edible fungi resistant to the disease caused by P. agarici.