A gear box, comprising: a case in which a gear chamber for storing oil, and a breather chamber separated from the gear chamber by a partition and located above the gear chamber, are formed; a plurality of rotation shafts (70, 73, 77), each of which is at least partially located in the gear chamber so as to be rotatable, the plurality of rotation shafts (70, 73, 77) extending leftward and rightward; and a plurality of gears (72, 75, 76, 79) located in the gear chamber and respectively provided on the plurality of rotation shafts (70, 73, 77); wherein: the partition has a breather chamber entrance formed therein for communicating the gear chamber and the breather chamber to each other; the case has a breather chamber exit formed therein for communicating the breather chamber and outside of the case to each other; and an inner wall of the case or the partition has a first rib and a second rib formed thereon, the first rib being located forward to the breather chamber entrance and at least partially located above at least one of the gears, and the second rib being located rearward to the breather chamber entrance and at least partially located above at least one of the gears; wherein: the plurality of rotation shafts (70, 73, 77) include a first rotation shaft located below at least a part of the breather chamber; the first rib is located above the first rotation shaft; the plurality of rotation shafts include a second rotation shaft; and the plurality of gears include a gear provided on the second rotation shaft; the second rotation shaft is located rearward to the first rotation shaft; characterized in that the inner wall of the case or the partition has a third rib formed thereon which is at least partially located above the gear provided on the second rotation shaft. a case in which a gear chamber for storing oil, and a breather chamber separated from the gear chamber by a partition and located above the gear chamber, are formed; a plurality of rotation shafts (70, 73, 77), each of which is at least partially located in the gear chamber so as to be rotatable, the plurality of rotation shafts (70, 73, 77) extending leftward and rightward; and a plurality of gears (72, 75, 76, 79) located in the gear chamber and respectively provided on the plurality of rotation shafts (70, 73, 77); the partition has a breather chamber entrance formed therein for communicating the gear chamber and the breather chamber to each other; the case has a breather chamber exit formed therein for communicating the breather chamber and outside of the case to each other; and an inner wall of the case or the partition has a first rib and a second rib formed thereon, the first rib being located forward to the breather chamber entrance and at least partially located above at least one of the gears, and the second rib being located rearward to the breather chamber entrance and at least partially located above at least one of the gears; the plurality of rotation shafts (70, 73, 77) include a first rotation shaft located below at least a part of the breather chamber; the first rib is located above the first rotation shaft; the plurality of rotation shafts include a second rotation shaft; and the plurality of gears include a gear provided on the second rotation shaft;