A slip suppression control system for a vehicle comprising: a monitored value detecting device (33-35, 45) 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 (33-35, 45) and plural thresholds (M1, M1a, M1b, M1c); and a controller configured to execute traction control for reducing a driving power of a drive wheel based on determination of the threshold determiner unit; a vehicle speed sensor configured to detect a vehicle body speed; and a gear position sensor configured to detect a transmission gear position of a transmission; wherein the plural thresholds (M1, M1a, M1b, M1c) have at least first and second thresholds (M1a, M1b), and the second threshold (M1b) is set larger than the first threshold (M1a); characterized in that the traction control includes an amount-change-based control such that a parameter corresponding to the driving power in a state where the traction control is being executed is made smaller in a case where the monitored value is larger than the second threshold (M1b) than in a case where the monitored value is larger than the first threshold (M1a) and smaller than the second threshold (M1b), the parameter including an ignition control parameter of an engine; the amount-change-based control is executed at least immediately after the traction control is initiated, and the driving power is reduced by ignition control in the amount-change-based control; and the threshold determiner unit is configured to change the first threshold according to the vehicle body speed detected by the vehicle speed sensor and the transmission gear position detected by the gear position sensor. a monitored value detecting device (33-35, 45) 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 (33-35, 45) and plural thresholds (M1, M1a, M1b, M1c); and a controller configured to execute traction control for reducing a driving power of a drive wheel based on determination of the threshold determiner unit; a vehicle speed sensor configured to detect a vehicle body speed; and a gear position sensor configured to detect a transmission gear position of a transmission; wherein the plural thresholds (M1, M1a, M1b, M1c) have at least first and second thresholds (M1a, M1b), and the second threshold (M1b) is set larger than the first threshold (M1a); characterized in that the traction control includes an amount-change-based control such that a parameter corresponding to the driving power in a state where the traction control is being executed is made smaller in a case where the monitored value is larger than the second threshold (M1b) than in a case where the monitored value is larger than the first threshold (M1a) and smaller than the second threshold (M1b), the parameter including an ignition control parameter of an engine; the amount-change-based control is executed at least immediately after the traction control is initiated, and the driving power is reduced by ignition control in the amount-change-based control; and the threshold determiner unit is configured to change the first threshold according to the vehicle body speed detected by the vehicle speed sensor and the transmission gear position detected by the gear position sensor.