PURPOSE: To increase pressure resistance by a method wherein a shoulder part is provided at a top opening of a bottomed cylindrical case extending to the exterior, a cover having a passage in communication with a filter is engaged therewith and the upper part of the top opening is pressed from the exterior so as to press fit a retaining ring over it.

CONSTITUTION: A filter element 20 is provided in a bottomed cylindrical case 10. A shoulder part 11 is provided at a top opening of the case 10 extending to the exterior. A cover 30 is formed into a cylindrical shape and provided with a passage 31 in communication with the filter element 20 and its lower cylindrical part 35 is extended into engagement with the shoulder part 11. The upper cylindrical part 36 of the cover 30 is formed to have a smaller diameter than that of the lower cylindrical part 35 thereof and an upper cylindrical ring part 13 is formed by pressing a cylindrical ring part 13' at the upper end of the case 10 diametrically with a roll 60. A retaining ring 50 is press fitted over the upper cylindrical ring part to bring its lower retaining ring part 51 in close contact with a lower cylindrical ring part 12 and press an upper retaining ring part 52 thereof in contact with the upper cylindrical ring part 13. In this manner, the cover will not be detached even if the pressure of fluid in the case becomes high with a higher pressure resistance.