A straddled vehicle comprising an internal combustion engine, the internal combustion engine comprising: a case; a crank shaft rotatably supported on the case; a first side wall arranged inside the case to partition the inside of the case into a first chamber and a second chamber; an oil pan provided on a bottom portion of the first chamber to store oil therein; an oil tank formed in the second chamber; a scavenge pump having a first intake port that communicates with the oil pan and a discharge port that communicates with the oil tank; and a feed pump having a second intake port that communicates with the oil tank, wherein an oil return hole that connects together the oil tank and the first chamber is formed in an upper portion of the first side wall with regard to an vehicle up-down direction (U-D), characterized in that the internal combustion engine includes a guide rib protruding from the first side wall toward the first chamber; and the guide rib is formed so as to guide oil, which flows into the first chamber from the oil tank through the oil return hole, into the oil pan, wherein an axis direction of the crank shaft is assumed to be a left-right direction with regard to a vehicle left-right direction (L-R) and a horizontal direction perpendicular to the axis direction of the crank shaft is assumed to be a front-rear direction with regard to a vehicle front-rear direction (F-Re), the oil return hole is located forward relative to an axis of the crank shaft with regard to the front-rear direction; and the guide rib includes a portion located forward relative to a front end of the oil pan and a portion located rearward relative to the front end of the oil pan with regard to the front-rear direction. a case; a crank shaft rotatably supported on the case; a first side wall arranged inside the case to partition the inside of the case into a first chamber and a second chamber; an oil pan provided on a bottom portion of the first chamber to store oil therein; an oil tank formed in the second chamber; a scavenge pump having a first intake port that communicates with the oil pan and a discharge port that communicates with the oil tank; and a feed pump having a second intake port that communicates with the oil tank, wherein an oil return hole that connects together the oil tank and the first chamber is formed in an upper portion of the first side wall with regard to an vehicle up-down direction (U-D), characterized in that the internal combustion engine includes a guide rib protruding from the first side wall toward the first chamber; and the guide rib is formed so as to guide oil, which flows into the first chamber from the oil tank through the oil return hole, into the oil pan, wherein an axis direction of the crank shaft is assumed to be a left-right direction with regard to a vehicle left-right direction (L-R) and a horizontal direction perpendicular to the axis direction of the crank shaft is assumed to be a front-rear direction with regard to a vehicle front-rear direction (F-Re), the oil return hole is located forward relative to an axis of the crank shaft with regard to the front-rear direction; and the guide rib includes a portion located forward relative to a front end of the oil pan and a portion located rearward relative to the front end of the oil pan with regard to the front-rear direction.