A treatment apparatus comprises a treatment chamber for performing a treatment for a workpiece W, a loading chamber connected to the treatment chamber, for loading and unloading a holding member which contains the workpiece into and from the treatment chamber, and an input/output chamber for inputting and outputting the workpiece to and from the loading chamber. The input/output chamber includes a cassette accommodating vessel port which holds a cassette accommodating vessel. The vessel is filled with clean air or an inert gas, and is airtightly closed. A cassette receiving mechanism is disposed below the port and lowers only the cassette of the vessel so as to receive the cassette. Thus, when the cassette is input and output to and from the outside of the treatment apparatus, the workpiece contained within the cassette is not exposed to the atmosphere in a working region. Consequently, it is not necessary to improve the cleanliness level of the atmosphere in the working region. As a result, the construction cost of the clean room and the operation cost thereof can be reduced. A clean air blowing device is disposed below the port. The clean air blowing device blows a side flow of clean air. Thus, the atmosphere at a position below the port where air tends to stay is circulated. Thus, since the surface of the workpiece loaded in the treatment chamber is exposed to the side flow, particles and the like that adhere thereto are purged.