An air intake structure is incorporated into a motorcycle having a body frame including a hollow head member provided in its front wall with an air inlet opening and supporting a steering shaft having an upper end holding a handlebar, and paired, right and left hollow main frame members extending rearward from the head member on right and left sides, respectively, of the motorcycle. The air intake structure includes an air cleaner disposed behind the head member, an inlet duct having an open front end opening forward, and an open rear end connected to the air inlet opening of the head member, an intake duct inserted in the hollow main frame member, respectively having a front part extended toward the head member in the hollow main frame member and having an open front end, and a rear part connected to the air cleaner. An air passage defined by the intake duct has a sectional area increased downstream with respect to the direction of air flow. The intake duct are internally provided with straightening plate, respectively.