A transmission system for a vehicle comprising: a clutch disposed in a power transmission path between an engine and a vehicle wheel; a transmission disposed in the power transmission path, a clutch actuator configured to switch the clutch between an engaged state and a disengaged state; a shift actuator configured to switch a shift stage of the transmission; a vehicle wheel speed sensor configured to detect a rotational speed of the vehicle wheel as a detected rotational speed; a sudden deceleration detecting unit configured to detect a predetermined sudden deceleration of the vehicle wheel based on an output signal of the vehicle wheel speed sensor; and a shift control unit programmed to execute an ordinary shift control (S2 to S9) of controlling the clutch actuator, and further controlling the shift actuator to shift to a speed adaptive shift stage associated in advance with a value of the detected rotational speed, when the sudden deceleration detecting unit detects the predetermined sudden deceleration of the vehicle wheel, a sudden deceleration adaptive shift control (S10 to S19) of controlling the clutch actuator, and further controlling the shift actuator to shift to a sudden deceleration shift stage which is lower than an actual shift stage at a time of detection of the predetermined sudden deceleration, and higher than the speed adaptive shift stage associated with the value of the detected rotational speed characterized in that the shift control unit is programmed to perform, in the sudden deceleration adaptive shift control, shifting to a shift stage that is in accordance with a value of the detected rotational speed at a predetermined time before the detection of the sudden deceleration by the sudden deceleration detecting unit. a clutch disposed in a power transmission path between an engine and a vehicle wheel; a transmission disposed in the power transmission path, a clutch actuator configured to switch the clutch between an engaged state and a disengaged state; a shift actuator configured to switch a shift stage of the transmission; a vehicle wheel speed sensor configured to detect a rotational speed of the vehicle wheel as a detected rotational speed; a sudden deceleration detecting unit configured to detect a predetermined sudden deceleration of the vehicle wheel based on an output signal of the vehicle wheel speed sensor; and a shift control unit programmed to execute an ordinary shift control (S2 to S9) of controlling the clutch actuator, and further controlling the shift actuator to shift to a speed adaptive shift stage associated in advance with a value of the detected rotational speed, when the sudden deceleration detecting unit detects the predetermined sudden deceleration of the vehicle wheel, a sudden deceleration adaptive shift control (S10 to S19) of controlling the clutch actuator, and further controlling the shift actuator to shift to a sudden deceleration shift stage which is lower than an actual shift stage at a time of detection of the predetermined sudden deceleration, and higher than the speed adaptive shift stage associated with the value of the detected rotational speed characterized in that the shift control unit is programmed to perform, in the sudden deceleration adaptive shift control, shifting to a shift stage that is in accordance with a value of the detected rotational speed at a predetermined time before the detection of the sudden deceleration by the sudden deceleration detecting unit.