prevented that is the present embodiment suppresses a local decrease in filter performance and thereby extends the lifetime of the filter 904 in the above the technology pertaining to the present disclosure is described based on a specific form of embodiment thereof however needless to say the embodiment merely provides a non limiting example of how the technology pertaining to the present disclosure may be implemented and it should be construed that the present disclosure encompasses in the spirit and scope thereof for example the modifications described in the following 1 in the embodiment a conical shape is discussed as one specific example of a shape of the portion of the air flow distributer 903 facing the outlet 902 b that reduces pressure loss occurring when the air comes in contact with the air flow distributer 903 however as long as the air flow distributer 903 is capable of reducing such pressure loss the portion of the air flow distributer 903 facing the outlet 902 b may have any shape forming a mountain shaped protrusion such as a pyramid shape a frustum shape and a circular truncated cone shape when making such a modification in order to achieve a further reduction in pressure loss a slope of the mountain shaped protrusion may be curved to form a concave shape 2 the portion of the air flow distributer 903 facing the outlet 902 b may have one or more through holes formed therein this allows the air discharged from the outlet 902 b to reach the area of the main surface of the filter 904 that faces the rear.