A relief valve for an oil filter device integrally formed with a check valve has an annular shape, and has a front end portion, a guiding portion, a sealing portion, a recess portion, and a groove portion. The relief valve is attached to a protector only by inserting a cylindrical protector end portion into the groove portion thereof. At this time, the recess portion is elastically deformed so that the sealing portion moves inwardly, while the sealing portion is elastically compressed by the protector end portion. As a result, the relief valve closes an oil flow passage defined in a wall of the protector end portion only by an elastic deformation stress thereof. The relief valve can be readily molded and attached to the oil filter device and stably provide desired valve opening characteristics.