Power unit for a small vehicle comprising: a gear chamber that is formed within a gear case attached to an engine body including a crankcase with a crank chamber formed therein and is isolated from the crank chamber; and a gear mechanism that has a drive gear to which power is transmitted from a crankshaft rotatably supported in the crankcase, a final gear mounted on an axle of a drive wheel, and counter gears (86, 87) arranged between the final gear and the drive gear and is contained within the gear chamber, wherein: a rotation axis of the final gear having a lower portion immersed in oil stored in a lower portion within the gear chamber is placed at a higher level than a rotation axis of the counter gears (86, 87); an oiling rib placed above the final gear and extending toward the final gear protrudes into the gear case; the gear case is provided with an oil guide wall formed in an arc shape along tooth tips of the final gear to guide oil stored in the lower portion within the gear chamber to the oiling rib; the gear case is attached to a rear portion of a power-transmission case adjoining the crankcase, extending toward a side of the drive wheel and containing a belt-type continuously variable transmission placed between the crankshaft and the drive gear, and a breather chamber extending in an up-and-down direction is formed above the drive gear and the counter gears (86, 87) and in front of the final gear in the gear case; the oiling rib placed between the breather chamber and the oil guide wall is hung down from a ceiling wall of the gear case and has a leading end placed near the tooth tips of the final gear; and the gear case includes a main case forming a part of the power-transmission case and integrally adjoining the crankcase, and a cover coupled to a rear portion of the main case and causing the gear chamber to be formed between the main case and the cover, one end of the axle is rotatably supported through a bearing member by the main case, characterised in that the rotation axis of the final gear is placed at a higher level than a rotation axis of the drive gear, the final gear is mounted on the axle in a position closer to the bearing member, and the main case is provided with a surrounding wall extending toward the final gear and surrounding the axle at least from below, and introduction walls (107, 108) placed below the oiling rib to guide oil toward the surrounding wall. a gear chamber that is formed within a gear case attached to an engine body including a crankcase with a crank chamber formed therein and is isolated from the crank chamber; and a gear mechanism that has a drive gear to which power is transmitted from a crankshaft rotatably supported in the crankcase, a final gear mounted on an axle of a drive wheel, and counter gears (86, 87) arranged between the final gear and the drive gear and is contained within the gear chamber, wherein: a rotation axis of the final gear having a lower portion immersed in oil stored in a lower portion within the gear chamber is placed at a higher level than a rotation axis of the counter gears (86, 87); an oiling rib placed above the final gear and extending toward the final gear protrudes into the gear case; the gear case is provided with an oil guide wall formed in an arc shape along tooth tips of the final gear to guide oil stored in the lower portion within the gear chamber to the oiling rib; the gear case is attached to a rear portion of a power-transmission case adjoining the crankcase, extending toward a side of the drive wheel and containing a belt-type continuously variable transmission placed between the crankshaft and the drive gear, and a breather chamber extending in an up-and-down direction is formed above the drive gear and the counter gears (86, 87) and in front of the final gear in the gear case; the oiling rib placed between the breather chamber and the oil guide wall is hung down from a ceiling wall of the gear case and has a leading end placed near the tooth tips of the final gear; and the gear case includes a main case forming a part of the power-transmission case and integrally adjoining the crankcase, and a cover coupled to a rear portion of the main case and causing the gear chamber to be formed between the main case and the cover, one end of the axle is rotatably supported through a bearing member by the main case, characterised in that the rotation axis of the final gear is placed at a higher level than a rotation axis of the drive gear, the final gear is mounted on the axle in a position closer to the bearing member, and the main case is provided with a surrounding wall extending toward the final gear and surrounding the axle at least from below, and introduction walls (107, 108) placed below the oiling rib to guide oil toward the surrounding wall.