To provide a swing arm structure capable of suppressing the required rigidity of a cross member to a low level to reduce the weight of the cross member. In a swing arm structure, a swing arm includes a pair of left and right arm bodies with a cross member spanning therebetween. Ends of the swing arm suspend rear wheels. A rear wheel-driving rear drive shaft, connected to a rear final reduction gear unit, is placed above the cross member. As viewed laterally, the cross member is contained within the profile of each arm body and is formed to be trapezoidal in section such that its bottom portion is broader than its upper portion.