Engine and saddle-riding type vehicle including the same

There is provided an engine in which a projection of the oxygen concentration sensor outwardly of the cylinder head can be prevented and engine size increase is reduced even if the oxygen concentration sensor is provided at a position at which the exhaust gas has a high temperature in the cylinder head. Also, there is provided a saddle-riding type vehicle which includes the engine. An engine 36 includes: a cylinder head 46 which has a projected portion 66 projecting outwardly from a head main body 65; a combustion recess 70 formed in the head main body 65; an exhaust gas passage 80 from the combustion recess 70 through the projected portion 66 for discharging exhaust gas from the combustion recess 70; and an oxygen concentration sensor 50 which has a main body portion 118 and a detection portion 120. The oxygen concentration sensor 50 is mounted to the projected portion 66. The main body portion 118 and the detection portion 120 overlap the projected portion 66 when viewed from the direction of a cylinder axis A. Further, at least part of the detection portion 120 is located within the exhaust gas passage 80.