Abstract: A subsidiary fuel tank apparatus in a motorized two-wheeled vehicle of the type that a vehicle body frame having front and rear wheels is provided with an internal combustion engine in the middle, a fuel tank on the upper side, and a detachable driver's seat in the rear of the fuel tank. The vehicle body frame is provided at its rear portion with a pair of right and left back stays situated below the driver's seat and with a pair of front and rear cross members extending across the back stays so that a room space is formed between the cross members and below the driver's seat, and a subsidiary fuel tank is accommodated in the room space.