Abstract: A fuel tank system of a motor bicycle according to the present invention comprises: a tank main body made of a synthetic resin, which is formed into a reverse U-letter shape in its longitudinal section in a manner of riding on a front head portion of a vehicle-body frame; a fuel supply port which is provided on an upper portion of the tank main body; a receiving pan portion which is provided in the fuel supply port; a relay joint which is arranged in a vicinity of a bottom portion of the tank main body and whose attaching direction is the substantially same as an attaching direction of the receiving pan portion; and a drain pipe which comprises a soft hose, passes through an interior of the tank main body, and connects between the receiving pan portion and the relay joint.