Abstract: A clean room having an air-conditioner unit built in the shell of an airtight main room in such a manner as to form part of the internal wall of the main room. This air-conditioner unit is provided with an external casing of a small thickness and this external casing is provided with an air inlet and an air outlet which is fitted with a high-performance filter. The casing has an air blower disposed therein. The airtight main room is mainly formed of a plurality of gate-shaped frames disposed in parallel as regularly spaced on a floor surface. The air-conditioner unit and a control unit are disposed in the spaces intervening between the adjacent vertical members of the gate-shaped frames. The internal wall of the main room is formed of wall panels and the front panel of the casing of the air-conditioner unit.