oil 34 is stored in the bottom of the transmission case 25 with output gears 5 7 10 12 14 18 and 24 being partially immersed in the lubricating oil 34 an auxiliary oil reservoir 35 is provided in an upper portion of the transmission case 25 and is spaced from said output gears the auxiliary oil reservoir 35 is comprised of a pair of oil reservoir halves 35 1 and 35 2 respectively said reservoir halves formed in abutting surfaces of the first and second case portions 25 1 and 25 2 respectively with opened surfaces of the halves being mated with each other with such a construction the auxiliary oil reservoir 35 is formed integrally with the transmission case 25 referring to fig2 and 4 the auxiliary oil reservoir 35 is connected via an oil passage 38 described hereinafter with an oil pump 36 pump 36 operates to pump up the lubricating oil 34 stored in the bottom of the transmission case 25 the bottom of reservoir 35 is provided with a small hole 39 which connects the inside of the auxiliary oil reservoir 35 with the interior of transmission case 25 the small hole 39 serves as a circulating means for regulating the amount of oil circulated from the auxiliary oil reservoir to the transmission case in the illustrated embodiment the oil pump 36 is constructed into a trochoidal type and is mounted on a pump housing 40 integral with the first case portion 25 1 a drive gear 42 is secured to a pump.