The sterilizing effect of particle irradiation on microorganisms for the sterilizing treatment thereof can be evaluated. The evaluation can be done by supplying microorganisms in the space inside a container (8), allowing particles (7) for the sterilizing treatment of microorganisms to irradiate the microorganisms, sampling the microorganisms by a sampling means (6) after the irradiation of the particles (7) and measuring the sampled microorganisms. The microorganisms as the subject for the sterilizing treatment can be a combination of one or more members selected from the group consisting of bacteria, mycete, viruses and allergens. As the particles, for example, positive ions, negative ions, and gases of positive ions and negative ions in mixture, charged particles such as α rays and β rays, various plasma gas particles, particles such as ozone and radical particles, and particles of chemical agent can be used.