A transmission control system comprising: a detection device arranged to detect a down-shifting operation; a throttle opening adjustment mechanism arranged to adjust an opening of a throttle valve; a shift mechanism (6, 7) arranged to change a gear position of a transmission; a clutch actuation mechanism arranged to switch a state of a clutch between a disconnection state and a transmission state; and a controller arranged to control the throttle opening adjustment mechanism, the shift mechanism (6, 7), and the clutch actuation mechanism; wherein the controller is arranged to perform, after the detection device detects the down-shifting operation, a first control operation to increase the opening of the throttle valve to a first value by operation of the throttle opening adjustment mechanism and a second control operation to switch the clutch to the disconnection state by operation of the clutch actuation mechanism; the controller is arranged to perform, after the clutch is switched to the disconnection state, a third control operation to change the gear position of the transmission by operation of the shift mechanism (6, 7) and a fourth control operation to decrease the opening of the throttle valve to a second value that is smaller than the first value and larger than a minimum value by operation of the throttle opening adjustment mechanism; and the controller is arranged to perform a fifth control operation to switch the clutch to the transmission state by operation of the clutch actuation mechanism after the opening of the throttle valve is decreased to the second value, wherein the controller is arranged to control, when performing the fourth control operation, the opening of the throttle valve such that the opening of the throttle valve is not smaller than the second value and is adjusted to the second value at the time point at which the clutch is switched from the disconnection state to the transmission state. a detection device arranged to detect a down-shifting operation; a throttle opening adjustment mechanism arranged to adjust an opening of a throttle valve; a shift mechanism (6, 7) arranged to change a gear position of a transmission; a clutch actuation mechanism arranged to switch a state of a clutch between a disconnection state and a transmission state; and a controller arranged to control the throttle opening adjustment mechanism, the shift mechanism (6, 7), and the clutch actuation mechanism; wherein the controller is arranged to perform, after the detection device detects the down-shifting operation, a first control operation to increase the opening of the throttle valve to a first value by operation of the throttle opening adjustment mechanism and a second control operation to switch the clutch to the disconnection state by operation of the clutch actuation mechanism; the controller is arranged to perform, after the clutch is switched to the disconnection state, a third control operation to change the gear position of the transmission by operation of the shift mechanism (6, 7) and a fourth control operation to decrease the opening of the throttle valve to a second value that is smaller than the first value and larger than a minimum value by operation of the throttle opening adjustment mechanism; and the controller is arranged to perform a fifth control operation to switch the clutch to the transmission state by operation of the clutch actuation mechanism after the opening of the throttle valve is decreased to the second value, wherein the controller is arranged to control, when performing the fourth control operation, the opening of the throttle valve such that the opening of the throttle valve is not smaller than the second value and is adjusted to the second value at the time point at which the clutch is switched from the disconnection state to the transmission state.