To reduce the use amount of lubricant so that the oil levels of the lubricant become a required level regardless of the combination of a plurality of speed change gears while using box bodies of the same size and shape.

In an inner space 5 of the box body 4 of this transmission 3, an input shaft 6 from an electric motor 2 is coupled to an output shaft 7 via gears 8a, 8b, 8c, 8d as speed change mechanisms, and the lubricant is injected to a degree so that the gears 8a, 8b, 8c are immersed therein. The sizes of the gears 8a, 8b, 8c, 8d differ according to the speed change ratio. When making the shape and size of the box bodies 4 common with respect to the transmission 3 having different speed change ratios such as ones of a series, when the gear 8c provided to a shaft 9 and connecting between the gears 8a and 8d is small, an extra space 13 occurs at the lower side of the gear 8c. By disposing and fixing a member 14 in the extra space 13, the usage amount of the lubricant corresponding to the volume of the member 14 is reduced.