PURPOSE: To prevent a transmission gear from breakage due to lowering of lubrication, by poviding oil reservoirs with the upper part and the lower part of the casing independently.

CONSTITUTION: The lubricating function to a lower bevel gear 27a, ahead and astern bevel gears 27b and 27c, etc. is spoilt during a small ship 40 is sailing, if the oil seal 26 etc. is broken by sand, etc. and the lubricant is gradually leaked and lost from the ship. The lubricant in the upper oil reservoir 11, however, will not be flown out, so that lubrication to the upper bevel gears 24a and 24b, etc. is maintained in good condition, therefore, the damage is reduced to the minimum. Further, the lubricant in the upper oil reservoir 11 is circulated by the circulating pump 29 driven by the drive shaft 22, as well as is cooled by the water of water tank 13 in the lower part of the upper oil reservoir 11 during circulating in it. As a result, lubricating function will not be deteriorated due to the rise of temperature of the oil.