Gas sterilization which complements the steam sterilization is used for the materials which are unstable under high temperature and moisture or items to which the steam sterilization is inapplicable for the structural reasons, therefore it is essential for medical institutions employing precise and delicate medical devices to use this kind of low temperature sterilizer. This sterilizer uses the 100% EO gas as sterilant and maintains the below atmospheric pressure during cycle processing, therefore gas does not leak to the outside chamber and also does not affect to the environment. 5 kinds of cycle program which has the different sterilizing temperature, time and aeration time are built in the sterilizer and operator can select a proper program easily according to the kind of load to be sterilized and sterilizing environments. The gas connection is very easy and convenient by inserting the small gas cartridge, charged with 100% EO gas and sealed, into the cartridge holder which is a recess in the chamber wall and then lock the door, it just all OK. Cycle processing such as conditioning, gas injecting, sterilizing, venting, aeration is fully automated by a controller adopted microprocessor and is monitored in real time during operating. The executing cycle is aborted automatically for safety, the error code displayed on the touch screen monitor accompanying audible alarm by the self diagnosis program if an error has been occurred during cycle processing.