VEHICLE AND SINGLE-CYLINDER FOUR-STROKE ENGINE UNIT

A vehicle on which a single-cylinder four-stroke engine unit which is able to stably detect the oxygen density in exhaust gas is mounted is provided. A single-combustion-chamber main catalyst 39 is at least partially positioned frontward of a crankshaft axis Cr1, and purifies the exhaust gas exhausted from one combustion chamber 29 most in an exhaust path extending from the one combustion chamber 29 to a discharge port 35e. The single-combustion-chamber main catalyst 39 is provided between an upstream bended portion 34c and a downstream bended portion 34e of a single-combustion-chamber exhaust pipe 34. A single-combustion-chamber upstream oxygen detector 37 is provided upstream of the single-combustion-chamber main catalyst 39 in the flow direction of the exhaust gas.