A gas is supplied to a second chamber so that the pressure in the second chamber is raised to a predetermined level. A communication passage is provided for internally connecting the first and second chambers. When the pressure in the first chamber attains the predetermined level, the gas is allowed to flow from the second chamber into the first chamber through the communication passage. A gas flow can be checked when an open-close door is opened to connect the chambers. Thus, there is no substantial gas flow, so that particles can be prevented from being flung up.