PURPOSE: To simplify the installation and removal of a filter element even if it is disposed in a small attaching space, by forming the opening surface of a case base part in an inclined shape to be opened in the direction of installation and removal of the filter element.

CONSTITUTION: In order to dispose a filter element 8 in a case base part 4 the filter element 8 is inserted into the opening part 26 of the case base part 4, directed in the direction of installation and removal of the filter element 8 with an element seal being abutted against one end of the filter element 8 while the latter is rotated in the direction of the arrow A, so that the filter element 8 is engaged with the partition wall of the case base part 4. Further, the other end of the filter element 8 is abutted against an element seal 14, and the short side section 6-b of a lid 6 is met with the long side section 4-b of the case base part 4. Further, a locking section at the front end of a clamp 30 is locked to the edge section 6-c, and is fastened by turning a manupulating member about a support shaft 36 toward the case base part 4.