To reduce the number of parts and the number of assembling processes.

A valve plug 40 consists of a valve member 41 and a compression coil spring 48. The valve member 41 is formed into a bottomed cylindrical shape and supported in a reciprocally movable manner by a small diameter cylindrical part 22 and force acts in the Z-direction shown by an arrow by the wt. of the valve plug 40, and a contact part 42 can be seated on the seat part 37 of a seat 35. When an engine is operated and the filter material 31 of an element 30 generates clogging, the pressure of the oil in a passage 25 becomes excessive and the valve member 41 is moved toward an inner bottom surface 22a to open a passage 24 by a slide part 43 to allow an intake part 1 to communicate with the passage 24. Since the valve plug 40 can achieve both of a function as a relief valve and a function as a check valve, the number of parts and the number of assembling processes can be reduced and the production cost of an oil filter 10 can be lowered.