To provide a dry filter medium for a bath water circulating and cleaning apparatus on which microorganisms having sporulability and protein and lipid decomposing activities are absorbed.

Microorganisms capable of being Bacillus subtilis Bn3 are adsorbed on a filter medium at a bacteria density S of 105 CFU/g filter material or more and the filter medium is dried up to a water content of 0.3 to 3%. A dry filter medium producing method, a bath water circulating and cleaning apparatus wherein the dry filter medium is included in a cleaning portion and a culture method of microorganisms adsorbed on the filter medium using the dry filter medium are also disclosed. By circulating hot water of about 80°C within the closed circuit of the bath water circulating and cleaning apparatus, general bacteria can be removed and, by accelerating the propagation of heat- resistant selected microorganisms, a necessary amt. for cleaning bath water of the cleaning microorganisms can be cultured for a shot time.