A transmission comprising: a gear train driven by a driving source; a case that supports the gear train inside; lubricant with which the case is filled; and an oil pan provided at a bottom of the case, wherein the oil pan includes a lubricant guide surface positioned above a lower end of a lowermost-level gear that is positioned at the lowest position in the gear train, the lubricant guide surface positioned above the lower end of the lowermost-level gear is inclined from an upper end of the lubricant guide surface to a lower end of the lubricant guide surface so as to descend toward the lowermost-level gear in an axial view of the lowermost-level gear, and is inclined so as to descend from opposite ends (67c, 67c) to a central part in the axial direction of the lowermost-level gear on the lubricant guide surface, wherein the case includes a side wall facing a tip of a driving source output shaft of the driving source, and the side wall includes an inclined portion that is inclined so as to be more separated in the axial direction from the driving source output shaft toward the oil pan on a lower side; wherein the gear train includes a counter gear that meshes with the driving source output shaft, and the lowermost-level gear that meshes with the counter gear, the counter gear integrally includes a driven gear portion that meshes with the driving source output shaft, and a driving gear portion that meshes with the lowermost-level gear, and at least a part of the driven gear portion is positioned inside the oil pan, wherein, the inclined portion is formed in a position overlapping with a side surface of an upper part of the driven gear portion in an axial view of the counter gear,wherein a guide rib protruding to an inside of the case is formed in the inclined portion, and the guide rib extends in an up-down direction, wherein the driving gear portion has a diameter smaller than that of the driven gear portion and is provided closer to the side wall side than the driven gear portion and wherein the guide rib is provided above the driving gear portion, and overlaps with at least one of the driven gear portion and a gear of the driving source output shaft in the axial view of the counter gear characterized in that a valley shape going downward from the opposite ends (67c, 67c) toward the central part is formed in a range from the upper end to the lower end and the lubricant guide surface is formed in a tub shape depressed such that the central part is the lowest, wherein the inclined portion is provided immediately above the driving gear portion of the counter gear, and the lubricant guide surface is provided immediately below the inclined portion. a gear train driven by a driving source; a case that supports the gear train inside; lubricant with which the case is filled; and an oil pan provided at a bottom of the case, wherein the oil pan includes a lubricant guide surface positioned above a lower end of a lowermost-level gear that is positioned at the lowest position in the gear train, the lubricant guide surface positioned above the lower end of the lowermost-level gear is inclined from an upper end of the lubricant guide surface to a lower end of the lubricant guide surface so as to descend toward the lowermost-level gear in an axial view of the lowermost-level gear, and is inclined so as to descend from opposite ends (67c, 67c) to a central part in the axial direction of the lowermost-level gear on the lubricant guide surface, the case includes a side wall facing a tip of a driving source output shaft of the driving source, and the side wall includes an inclined portion that is inclined so as to be more separated in the axial direction from the driving source output shaft toward the oil pan on a lower side; the gear train includes a counter gear that meshes with the driving source output shaft, and the lowermost-level gear that meshes with the counter gear, the counter gear integrally includes a driven gear portion that meshes with the driving source output shaft, and a driving gear portion that meshes with the lowermost-level gear, and at least a part of the driven gear portion is positioned inside the oil pan, a guide rib protruding to an inside of the case is formed in the inclined portion, and the guide rib extends in an up-down direction, wherein the driving gear portion has a diameter smaller than that of the driven gear portion and is provided closer to the side wall side than the driven gear portion and characterized in that a valley shape going downward from the opposite ends (67c, 67c) toward the central part is formed in a range from the upper end to the lower end and the lubricant guide surface is formed in a tub shape depressed such that the central part is the lowest, wherein the inclined portion is provided immediately above the driving gear portion of the counter gear, and the lubricant guide surface is provided immediately below the inclined portion.