A transmission control system includes an ECU, a throttle actuator, a clutch actuator, and a shift actuator. The throttle actuator adjusts a throttle opening under control of the ECU, the clutch actuator adjusts a state of a clutch under control of the ECU, and the shift actuator changes a gear position of a transmission under control of the ECU. When the transmission is shifted down, a throttle opening is adjusted to a first value and the clutch is disconnected such that a rotational speed of an engine increases. Then, the gear position of the transmission is moved to a lower position by one stage with the clutch being disconnected. After that, the throttle opening is decreased to a second value, and connection of the clutch is started. The second value is set to a value larger than a minimum value.