PURPOSE: To provide a new method for fumigation in chamber capable of simply fumigating a chamber without requiring use of a chemical difficult to handle and complicated operation such as sealing up a window.

CONSTITUTION: An ozonizer 1 and an ozone decomposing device 2 are laid in a chamber and the chamber is fumigated with ozone produced from the ozonizer 1 for a specific time. The produced ozone in the chamber is decomposed by the ozone decomposing device 2 into oxygen. The oxygen is exhausted out of the chamber and the chamber is made in a negative pressure state.