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 enrichment of degrading bacteria, which contains a porous material provided as a support on which degrading bacteria A capable of degrading at least one organic contaminant and degrading bacteria B capable of degrading another organic contaminant are enriched, is produced. The degrading bacteria A may be a PCNB-degrading bacteria, particularly degrading bacteria containing degrading bacteria having part or all of the bacteriological characteristics of Nocardioides sp. PD653 and the degrading bacteria B may be degrading bacteria containing degrading bacteria having part or all of the bacteriological characteristics of .beta.-Proteobacteria CDB21.