A method of controlling speed change of a hydraulic drive device for vehicles and a speed change device, which provide a favorable travelling efficiency and a favorable controllability. The method of controlling speed change comprises performing control through discriminating at least between power running and brake running from a magnitude of acceleration (θ) and a rotating speed of a hydraulic motor (50). At the time of power running, a change-over valve (21) may be fully opened in accordance with the rotating speed (ωout) of the hydraulic motor (50) and the magnitude of acceleration (θ). Also, the speed change device comprises a detecting sensor (61a) for the magnitude of acceleration, a motor rotating speed detecting sensor (52) for detecting a vehicle speed (V), and a control device (60) for discriminating between power running and brake running on the basis of the magnitude of acceleration (ωout) and the vehicle speed (V) for controlling.