When a first floor (11) which is formed of a punching plate or the like and through which air passes is provided immediately below an arm (17) at a middle height part of a conveying robot (10) in a casing (2a) of a clean transfer device (2) and a degree of opening of a casing bottom part frame (2b), which supports a base part of the conveying robot (10), with respect to the outside is restricted, a class 1 can be maintained. Here, when a second floor (13) formed of a punching plate or the like is used on the casing bottom part frame (2b), a class 0 state can be realized under specific conditions, thereby enabling production of a semiconductor having a wire width of 0.1 .mu.m. As a result, the device can cope with the unexpectedly high degree of cleanliness of 0.1 .mu.m particle class 1, which cannot be realized in the prior art, requested also for the transfer device according to a reduction in wire width on a highly integrated semiconductor wafer.