A condensation sensor detects whether hydrogen peroxide gas has condensed in a sealed space and determines the condition of a condensate film upon passage of time. The condensate film in the sealed space is controlled with the condensation sensor. The condensation sensor includes a condensate forming part with a plurality of glass plates arranged so that a direction of irradiation is substantially perpendicular to a surface direction, and the condensate forming part is disposed between a projector and a light receiver. The condensation sensor is placed inside an isolator and irradiated with laser beams. The condensate film formed on the glass plates is detected from a change in the quantity of light received by the light receiver, thus the condition of condensation on the surface of an item to be sterilized inside the isolator is presumed, and accordingly, the necessary and sufficient amount of gas introduced is determined.