PURPOSE: To establish and maintain an environment independent from surrounding environments within a mobile unit by taking in air from outside, and removing contaminated particles and contaminated gas by filtration, and passing the air into the mobile unit so that it may make specified pressure higher than the outside air pressure.

CONSTITUTION: When specified air pressure is achieved, the air filtrated by a filter unit 16 is mixed with the air recirculated from within a mobile unit 10, and this mixing of air is continued so long as the air pressure within the mobile unit 10 is the same as the specified pressure or higher than this. This mixed air is heated or cooled to a specified temperature, and is distributed substantially equally into the mobile unit 10, and is filtrated with a filter unit 16 so as to remove remaining particles and gas. Hereby, environment independent from the surrounding environment can be established and maintained, concerning both of temperature and pollution, within a sealed tent or a similar mobile unit 10.