Abstract: In semiconductor production plants a highly clean state is required. In a plant building, through which air is forcibly circulated, a clean room in a low or medium cleanliness state is formed using a filter. A plurality of processing units are provided in the clean room and held in a high cleanliness state. A transport robot transfers a workpiece or like object to and from each processing unit. The robot is capable of being driven to positions corresponding to the processing units and holds the workpiece or object in a highly clean state. Thus, there is no need of holding the entire building highly clean, and only a required part of the building may be held in a highly clean state, which is advantageous from the standpoint of cost.