A transmission control system is capable of promptly compensating for a disturbance attributable to a mechanical interference that takes place in a selecting operation so as to quickly complete the selecting operation. An adaptation disturbance observer provided in a selection controller calculates a disturbance component term in a model equation of a selecting mechanism for each control cycle of the selection controller. An equivalent control input calculator of a sliding mode controller uses the disturbance component term calculated by the adaptation disturbance observer to calculate an equivalent control input. An adder adds a reaching law input calculated by a reaching law input calculator and the equivalent control input to calculate a control value of a voltage to be applied to a selection motor of the selecting mechanism.