A cassette adapter which makes it possible for a second cassette which accommodates a circular wafer of a diameter of 200 mm to be used on a load port ready for a first cassette which accommodates a different circular wafer of another diameter of 300 mm is locked to the load port by an adapter main body locking apparatus. The locking apparatus includes a locking air cylinder serving as a locking member for locking the first cassette to a carrier base of the load port, and a locking target member provided integrally on a rear face of an adapter plate of the cassette adapter for disengageably engaging with a locking pawl at an end of a rod of the air cylinder. A seating sensor mechanism detects that the second cassette is seated at a correct position on the adapter plate.