A filter portion (10) includes a casing 12, a closing plate (14), a reinforcing plate (16), an element holding plate (17), a sensor holding plate (18) and an element (20). The element holding plate (17) is formed in a cylindrical shape with the bottom having the inside diameter being substantially equal to the outside diameter of the element (20) to hold the end portion of the element (20) by the side wall thereof. On the bottom portion of the cylindrical element holding plate (17) are formed three tapered portions (17a) axially extending to the opposite side thereof so as to positionally correspond to the respective through holes (16b) provided in the reinforcing plate (16). At the tip end of each tapered portion 17a is formed a tube portion (17b) with the outside diameter being roughly equal to the inside diameter of the through hole 16b, and the tip end portion (17c) of each tube portion (17b) is fixed to the reinforcing plate (16) by means of caulking. In this way, it is possible to reduce the size of the frame of the housing thereof.