At a preparing stage, a tool is placed in contact with an inner wall of a rotation transmitting portion of an adjustment screw. Then, at an adjustment rotation amount calculating stage, a pre-adjustment oil pressure, which is an output oil pressure of an oil pressure control valve at a pre-adjustment rotational position of the adjustment screw under a predetermined drive condition of a drive device, is sensed, and thereafter an adjustment rotation amount of the adjustment screw from the pre-adjustment rotational position to a target rotational position of the adjustment screw, which corresponds to a target oil pressure, is calculated based on: a differential pressure between the pre-adjustment oil pressure and the target oil pressure; and a correction amount which corresponds to a product characteristic of the oil pressure control valve. Then, at a rotating stage, the adjustment screw is rotated by the adjustment rotation amount.