Many small air cleaners for business purposes with HEPA filter are used in microelectronics industries and biol. fields.In the biol. field, such as hospital or laboratory, the air cleaner corrected unspecified germs that may have a potential high risk of infection.At the maintenance work, we should sterile the air cleaner body and filters in short time, safely, simply, and should be reliable to protect the stuff from infection.In this study, we made a prototype formaldehyde gas sterilizer, which can sterile whole unit of small air cleaner, before the HEPA filter replacement.The sterilizer can also neutralize the formaldehyde gas before exhaust.At first, we examined the concentration of formaldehyde gas and sterility effect according to the manuals of biol. safety cabinet such as NSF standard Number49 annex G. and JIS K-3800 (Japan industrial standard).We observed very low, approx. 1/7, evaporated gas concentration compare to theory (8000 ppm).We think it is important to monitor the gas concentration during the decontamination of the safety cabinet, not only evaporate the calculated amount of paraformaldehyde.The next, we tested the sterility assurance level (SAL) by the Biol. Indicator (BI) strip, under various times and concentrations of gas fumigation.To sterile BI strip, the fumigation should exceed average 1200 ppm for 6 h .We think the SAL is 7200 ppmhr.If we want to shorten the sterilization time, the concentration must be higher but would increase risk of leakage.Addnl., we confirmed how effective and reliable the procedure and the condition of 7200 ppmhr for actual air cleaner that were operated for 55 days in our facility.We will present the results of our experiments