PROBLEM TO BE SOLVED: To provide a bacterium (decomposition bacteria) decomposing an amino-s-triazine-based compound and cyanuric acid as a final metabolite thereof which are included in fertilizers, feeding stuff, industrial waste materials, industrial waste water, environmental water, or soils.SOLUTION: A decomposition bacterium has some or all of mycological characteristics which Arthrobacter sp. MCO lines or Arthrobacter sp. CSP lines have. A decomposition bacteria-keeping simple substance or microbial formulation comprises these decomposition bacteria in a porous material such as wooden carbonization material. These decomposition bacteria or the like can decompose amino-s-triazine-based compounds and cyanuric acid.