A Petri dish (S) storing therein a culture medium (K) with a specified thickness is supported on the dish support (17) of a casing (14) and then a nozzle holding member (13) is fitted to the upper portion of the casing (14). When a motor (19) is driven to rotate a high static pressure fan (18), air introduced through the hole (12a) of a nozzle (15) flows through the space between the nozzle (15) and the medium (K); a high trapping efficiency can be then maintained with the distance between the medium and the nozzle set to 0.5 to 1.5 mm.
