PURPOSE: To reduce the amount of dust leaking outward, and prevent the contamination of a wafer surface due to dust, by providing a wheel and a bottom surface with covers.

CONSTITUTION: A plurality of wheel covers 4 and a plurality of bottom surface covers 5 are provided. Those cover the upper surface of wheels except a wheel 2 of one end-portion of a main body part 1, and are fixed to the main body part 1. These are fixed to the bottom surface of the main body part 1 between the wheels 2. The wheel covers 4 cover the wheel 2 in the manner in which the external upper surface of the wheel 2 with an external diameter of about 45mm is covered with a several mm gap. The bottom surface covers 5 cover about 1/4 part of the lower side part of the external pheriphery of a wafer 8 with an external diameter of 6 inches. Therefore, dust generated by the wheel 2 can be prevented from attaching to the wafer 8 mounted on a boat 7 by the wheel covers 4. The dust blown up by travelling can be prevented from attaching to the wafer 8 by the existence of the bottom surface covers 5.