To allow sterilization gas to go around an operation room in short time during sterilization.SOLUTION: An isolator has an operation room, and a front door provided at the front of the operation room. With a circulation fan, through an air filter and a punching plate, the operation room is supplied with clean air. In a region between the air filter and the punching plate, at least two sterilization gas inlets are provided.SELECTED DRAWING: Figure 4