An oil filter unit comprising a housing (4) and a filter element (14) accommodated therein is fitted to an engine (1) via a bracket (3) having a cylindrical holder (5). The housing (4) is secured to the holder (5) by screwing a male screw (19) on its outer circumference to a female screw (17) formed in the inner circumference of the holder (5). A first cut-out (24) is formed across the female screw (17) and a second cut-out (23) is formed across the male screw (19). When these cut-outs (24, 23) meet together, they form a drain passage which drains the oil in the housing (4) to outside. With this construction, the oil in the housing is drained via a predetermined path and prevented from scattering around, when the filter unit is removed from the engine (1).