A filter housing is adapted to be assembled with a cap and installed with an O-ring. The filter housing includes a surrounding wall that is formed with a retaining groove for installation of the O-ring therein, and that has a plurality of protrusions. The retaining groove is defined among inner and outer surrounding surfaces and a groove end surface. The protrusions protrude from one of the inner and outer surrounding surfaces toward the other. The one of the inner and outer surrounding surfaces is adapted to be separated from the O-ring when the surrounding wall is separated from the cap, and is adapted to contact the O-ring when the surrounding wall is assembled with the cap and when the O-ring is squeezed by the cap to deform.