To improve freedom of layout inside an engine room and also improve sound absorbing effect by arranging an inlet side passage lead from a pump inlet to a pump flow passage inlet, an outlet side passage lead from a pump flow passage outlet to a pump outlet, and surrounding the passages by means of sound absorbing members.

Sound absorbing members 15, 16, 17 are surrounded by a casing 2 and a cover 13, while a filter 14 is arranged on a pump inlet 7. Dusts are removed from air suctioned into an electrically driven air pump from the outside after passages through the filter 14. The air, after passing through the filter 14, is introduced to a pump flow passage inlet 8 through an inlet side passage 18 surrounded by the sound absorbing members 15, 16, 17. Sound absorbing effect can be further improved by optionally varying a sectional area at an optional portion of the inlet side passage 18 surrounded by the sound absorbing member 16. Fine dusts which cannot be removed by the filter 14 are more effectively adsorbed by an outer peripheral part of the sound absorbing member 16.