There is provided a mini environment type transfer unit which can efficiently transfer a sample to a critical dimension scanning electron microscope (CD-SEM) even in the case of use of a SMIF pod which can store only one photomask. In addition to a load port, a stocker which can store a plurality of photomasks is provided in the mini environment type transfer unit. A mask storage slot in which a plurality of storage units are stacked is provided in the stocker, and one photomask is stored in each storage unit. A sensor is provided in each storage unit to determine whether or not the photomask is normally stored. Additionally, a sensor is provided in each storage unit to detect whether or not the photomask exists.