An oil filter attaching structure for a vehicle-mounted internal combustion engine, in which a crankshaft is supported in a crankcase so as to extend in vehicle widthwise directions, and in which the internal combustion engine has an oil filter for filtering engine hydraulic oil and installed in a filter case integrally formed with a crankcase cover of a crankcase, wherein: said crankcase cover is mounted on a side surface of said crankcase so as to cover an end of the crankshaft; an internal combustion engine accessory and a clutch device of an engine transmission are disposed inside of said crankcase cover in respective positions forward and rearward of said crankshaft; and a first oil passage formed in a wall of the crankcase cover for conducting hydraulic oil into the oil filter and a second oil passage for discharging filtered hydraulic oil from the oil filter, the second oil passage being connected to an oil reservoir defined inside the crankcase cover so as to guide filtered hydraulic oil into the oil reservoir, and wherein an end of the crankshaft is positioned in the oil reservoir to be supplied with hydraulic oil thereinto, characterized in that said oil filter and an oil supply port for the internal combustion engine are disposed in said crankcase cover in respective positions downward and upward of said crankshaft, and in that said second oil passage extends into the oil reservoir between the internal combustion engine accessory and the clutch device, a branched oil passage for guiding hydraulic oil to an upper portion of the internal combustion engine being formed to extend upward from the oil reservoir in a wall of the crankcase cover between the internal combustion engine accessory and the clutch device, and in that the oil supply port is positioned on an outer wall of the crankcase cover between the internal combustion engine accessory and the clutch device. said crankcase cover is mounted on a side surface of said crankcase so as to cover an end of the crankshaft; an internal combustion engine accessory and a clutch device of an engine transmission are disposed inside of said crankcase cover in respective positions forward and rearward of said crankshaft; and a first oil passage formed in a wall of the crankcase cover for conducting hydraulic oil into the oil filter and a second oil passage for discharging filtered hydraulic oil from the oil filter, the second oil passage being connected to an oil reservoir defined inside the crankcase cover so as to guide filtered hydraulic oil into the oil reservoir, and wherein an end of the crankshaft is positioned in the oil reservoir to be supplied with hydraulic oil thereinto, characterized in that said oil filter and an oil supply port for the internal combustion engine are disposed in said crankcase cover in respective positions downward and upward of said crankshaft, and in that said second oil passage extends into the oil reservoir between the internal combustion engine accessory and the clutch device, a branched oil passage for guiding hydraulic oil to an upper portion of the internal combustion engine being formed to extend upward from the oil reservoir in a wall of the crankcase cover between the internal combustion engine accessory and the clutch device, and in that the oil supply port is positioned on an outer wall of the crankcase cover between the internal combustion engine accessory and the clutch device.