To make it possible to easily and automatically clean a filter by moving the filter in connection with door-opening and wiping the surface of the filter with a brush located near the filter.

In maintenance and the like, a back door l8 opening and closing on a hinge 19 is opened to move a filter bracket 7 linked to the back door 18 in a rear direction along a groove 4c formed between a fan bracket 4 and a lid 8, and a filter 9 stuck to the filter bracket 7. The movement of the filter bracket 7 and the filter 9 in a rear direction will not come off from the width of a fan 5. A dust and the like stuck during a device operation is stripped off by a plastic antistatic brush for dust removal fixed to the both side of the fan bracket 4 every opening and closing of the back door 18. The dust will drop below without sticking to the plastic antistatic brush.