A small vehicle with a seat structure, comprising: a body frame, a seat (25, 70) located above and supported by the body frame to be opened or closed; a body cowl covering an under-seat region located under the seat; a fuel tank disposed in the under-seat region, supported by the body frame, and adapted to store a gas fuel lower in specific gravity than air, a gaseous fuel reactor disposed in the under-seat region to cause the gas fuel to undergo a chemical reaction; and a gas fuel sensor disposed in the under-seat region to detect gas fuel leaked from components related to the gaseous fuel reactor including the gaseous fuel reactor and the fuel tank, characterized in that the seat includes an under-seat curved bent portion surrounded at least by a seat base (55, 71) which is a bottom plate of the seat so as to provide a larger space in the under-seat region, if an uppermost point of a defining portion is designated as a point T, the defining portion protruding downward along a vehicle body from the seat base (55, 71) to define a substantially dome-shaped depression (62, 72) in a part of the under-seat curved bent portion, the gas fuel sensor is located in the depression (62, 72) and at least a part of the gas fuel sensor is arranged above a horizontal line passing through the point T, a plurality of reinforcement ribs (64, 73) are disposed to a lower surface of the seat base (55, 71) so as to extend downward therefrom, the reinforcement ribs (64, 73) extend in parallel to each other without intersecting each other substantially in a front-rear direction of the seat (25, 70) to thereby define concave grooves to the lower surface of the seat base (55, 71) so as to open downward, and the concave grooves defined by the reinforcement ribs extend substantially in the front-rear direction, the concave grooves defined by the plural reinforcement ribs are arranged in continuous to a location site of the gas fuel sensor. a body frame, a seat (25, 70) located above and supported by the body frame to be opened or closed; a body cowl covering an under-seat region located under the seat; a fuel tank disposed in the under-seat region, supported by the body frame, and adapted to store a gas fuel lower in specific gravity than air, a gaseous fuel reactor disposed in the under-seat region to cause the gas fuel to undergo a chemical reaction; and a gas fuel sensor disposed in the under-seat region to detect gas fuel leaked from components related to the gaseous fuel reactor including the gaseous fuel reactor and the fuel tank, characterized in that the seat includes an under-seat curved bent portion surrounded at least by a seat base (55, 71) which is a bottom plate of the seat so as to provide a larger space in the under-seat region, if an uppermost point of a defining portion is designated as a point T, the defining portion protruding downward along a vehicle body from the seat base (55, 71) to define a substantially dome-shaped depression (62, 72) in a part of the under-seat curved bent portion, the gas fuel sensor is located in the depression (62, 72) and at least a part of the gas fuel sensor is arranged above a horizontal line passing through the point T, a plurality of reinforcement ribs (64, 73) are disposed to a lower surface of the seat base (55, 71) so as to extend downward therefrom, the reinforcement ribs (64, 73) extend in parallel to each other without intersecting each other substantially in a front-rear direction of the seat (25, 70) to thereby define concave grooves to the lower surface of the seat base (55, 71) so as to open downward, and the concave grooves defined by the reinforcement ribs extend substantially in the front-rear direction, the concave grooves defined by the plural reinforcement ribs are arranged in continuous to a location site of the gas fuel sensor.