The present invention provides a stable complex microbial system, which simultaneously decomposes a plurality of organic contaminants even under a polluted environment with these contaminants and permits more effective decomposition of persistent organic contaminants such as PCNB and simazine. A support for holding a complexed accumulation of degrading bacteria, which contains a porous material provided as a support on which a degrading bacterium A capable of degrading at least one organic contaminant and a degrading bacterium B capable of degrading another organic contaminant are accumulated, is produced. The degrading bacterium A may be a PCNB-degrading bacterium, particularly a degrading bacterium containing a degrading bacterium having part or all of mycological characteristics of Nocardioides sp. PD653 and the degrading bacterium B may be a degrading bacterium containing a degrading bacterium having part or all of mycological characteristics of β-Proteobacteria CDB21.