To apply special contrivances to the structure and layout of a reserve oil tank disposed on the front side of a dry sump type internal combustion engine mounted on a snowmobile, thereby to achieve effective utilization of space in the snowmobile, and to achieve a further reduction in the size of the snowmobile. A reserve oil tank is laid out on the front side of a dry sump type internal combustion engine mounted on a snowmobile, and the layout of the tank is so made that a tank wall in the longitudinal direction crosses the engine front side. The tank is provided in a tank front wall in the longitudinal direction thereof with an arcuate recessed groove extending vertically in the tank, and a steering shaft vertically passes through the inside of the recessed groove.