In summer or on humid days, municipal solid waste, which is a raw material of RDF, rotts easily and causes bad odor, which makes the operation circumstance troublesome. In this study, CaO addition to the waste before the RDF production process is proposed, and the effect of CaO addition on the rotted material is experimentally examined. A mixture of soybean protein and distilled water was used as a model refuse. Escherichia coli was adopted as an indicator of rotting. The samples were put in a glove box for a few days, and then a constant amount of CaO powder was added to the samples. The changes in cell number of E. coli and pH with time were measured. CaO addition is effective for rotting suppression. However, as CaO addition is delayed, the higher content of CaO is required for rotting suppression. In the case of higher moisture content, the higher CaO content is required for rotting prevention.