PURPOSE: To lengthen the service life of collecting dirt and dust significantly by means of respective dust collecting filters by providing the first dust collecting filter to capture dirt and dust of comparatively coarse particles and the second dust collecting filter to capture fine dirt and dust in an air ventilating passage formed between an air intake port and a blower fan.

CONSTITUTION: The first dust collecting filter 11 and the second dust collecting filter 12 are provided in series in an air flowing direction in an air ventilating passage 10 formed between an air intake port 3 and a blower fan 7. In this case, the first dust collecting filter 11 captures dirt and dust of large diameter particles, and the second dust collecting filter 12 captures dirt and dust of small diameter particles passed through the first dust collecting filter 11. Thereby, the dirt and dust having respective particle diameters can be captured by means of both the first and the second dust collecting filters 11 and 12, the service life of capturing the dirt and dust can be lengthened about twice in the respective filters 11 and 12, so that maintenance work of the filters 11 and 12 and so on can be reduced by half.