Seat structure for small vehicle

A vehicle such as a motorcycle includes a seat (25), a body cowl covering an under-seat region, a fuel tank (42) disposed in the under-seat region to store a gas fuel lower in specific gravity than air, a gaseous fuel reactor (41) disposed in the under-seat region, and a gas fuel sensor (52). The seat includes an under-seat curved bent portion surrounded at least by a seat base (55) so as to provide a larger space in the under-seat region, and if an uppermost point of a defining portion (63) is designated as a point T, the defining portion (63) protruding downward along a vehicle body from the seat base (55) to define a substantially dome-shaped depression (62) in a part of the under-seat curved bent portion, the gas fuel sensor (52) is located in the depression (62) and at least a part of the sensor (52) is arranged above a horizontal line passing through the point T.