A snowmobile comprises a left-right pair of skis, an engine and a track belt provided on a vehicle body. The track belt is driven by the engine, and the skis are steered by an operation on a steering handlebar. The vehicle body frame is formed by connecting intermediate frame sections, which are U-shaped in side view, to a front portion of a rear frame section, the rear frame section having a roughly U-shaped section and connecting a front frame section to front portions of the intermediate frame sections. In the vehicle body frame, detachable side frame sections are used to connect the portions between the upper ends of front suspension members for supporting the skis and a front portion of a rear cushion for supporting the track belt. The reinforcing side frame sections include portions which are protuberant to the upper side and curved downwards, respectively. The resulting snowmobile frame structure is rigid enough to accommodate an engine of increased weight.