A swing arm support structure is provided in a motorcycle, in which first and second support arm portions are provided on a front portion of a swing arm. The support arm portions are pivotally supported by a power unit through first and second pivot shafts, so that rigidity and strength of the paired support arm portions is ensured. A flange is formed on the first pivot shaft, which extends at least partially up to an outer edge of a power unit case and projects radially outwardly from an end portion of the first pivot shaft adjacent the case. The flange portion is removably attached to the case. An outer end of an output shaft extends through and is supported by the first pivot shaft, and this outer end projects outwards of the first support arm portion. A drive gear is fixed to the outer end portion of the output shaft.