A slip suppression control system for a vehicle comprising: a monitored value detecting device for detecting a monitored value corresponding to a difference between a rotational speed of a front wheel of the vehicle and a rotational speed of a rear wheel of the vehicle; a threshold determiner unit configured to determine a relationship between the monitored value detected by the monitored value detecting device and a threshold; characterized in that the system further comprises a traction control unit configured to execute traction control including initial traction control and continued traction control executed continuously after the initial traction control, based on determination of the threshold determiner unit; wherein the traction control unit is configured to initiate the initial traction control for reducing a driving power of a drive wheel when the threshold determiner unit determines that the monitored value exceeds a first slip threshold; wherein the traction control unit is configured to execute the continued traction control such that the driving power is reduced when the threshold determiner unit determines that the monitored value exceeds a second slip threshold which is smaller than the first slip threshold, while the driving power is increased when the threshold determiner unit determines that the monitored value is smaller than a gripping threshold which is the second slip threshold or smaller; wherein a reduction amount of the driving power in the case where the monitored value exceeds the first slip threshold in the initial traction control is set larger than a reduction amount of the driving power in the case where the monitored value exceeds the second slip threshold in the continued traction control; and wherein the traction control unit is configured to execute a driving power reducing control to set a control command value to a predetermined value in the initial traction control, and to feedback control the driving power to cause the monitored value to be closer to the second slip threshold in the continued traction control. a monitored value detecting device for detecting a monitored value corresponding to a difference between a rotational speed of a front wheel of the vehicle and a rotational speed of a rear wheel of the vehicle; a threshold determiner unit configured to determine a relationship between the monitored value detected by the monitored value detecting device and a threshold; characterized in that the system further comprises a traction control unit configured to execute traction control including initial traction control and continued traction control executed continuously after the initial traction control, based on determination of the threshold determiner unit; wherein the traction control unit is configured to initiate the initial traction control for reducing a driving power of a drive wheel when the threshold determiner unit determines that the monitored value exceeds a first slip threshold; wherein the traction control unit is configured to execute the continued traction control such that the driving power is reduced when the threshold determiner unit determines that the monitored value exceeds a second slip threshold which is smaller than the first slip threshold, while the driving power is increased when the threshold determiner unit determines that the monitored value is smaller than a gripping threshold which is the second slip threshold or smaller; wherein a reduction amount of the driving power in the case where the monitored value exceeds the first slip threshold in the initial traction control is set larger than a reduction amount of the driving power in the case where the monitored value exceeds the second slip threshold in the continued traction control; and wherein the traction control unit is configured to execute a driving power reducing control to set a control command value to a predetermined value in the initial traction control, and to feedback control the driving power to cause the monitored value to be closer to the second slip threshold in the continued traction control.