The invention provides a vehicular opening/closing body control device capable of reducing braking load when a braking force is applied to an opening/closing body during the traveling of a vehicle. Aslide door (4), which serves as an opening/closing body, is opened/closed along a movement path (L) which is formed by a guide rail (10) extending in a longitudinal direction of the vehicle and a guide roller (30) engaged with the guide rail (10). In addition, the guide rail (10) includes a curved portion ([alpha]), which is curved in a width direction of the vehicle in a closing operation side end portion (10F), which is disposed in a closing operation direction of the slide door (4). In addition, a door ECU, which serves as a position detection unit, a traveling state determination unit, and a braking control unit, executes the braking control for applying the braking force to the slide door (4) when the opening/closing operation position of the slide door is within the specific range [beta], which is set in the movement path (L) of the slide door (4) so as to correspond to the curved portion ([alpha]) of the guide rail (10) during the traveling of the vehicle.