A fuel tank disposed between a head pipe and a rider's seat is provided, which includes a pair of grooved portion that are positioned on a top surface in symmetrical relation with respect to a longitudinal center plane. The pair of the grooved portions are positioned rearwardly of a tank top portion, each so formed as to have a transverse sectional shape that is represented by a smooth curved line. The grooved portions are parallel to the longitudinal center plane or inclined to extend with a distance therebetween gradually decreasing towards a rear, the distance being set to be within the range of 0.25 to 0.40 at a front end of each of the grooved portions and within the range of 0.08 to 0.35 at a rear end thereof.