A transmission control apparatus includes a first control board including a first control unit with an input/output port and a first board connector; and a second control board including a second control unit with an input/output port and a second board connector. The first control board is connected to the second control board via a general purpose cable, the general purpose cable is used for transmission of an indication signal as a digital signal controlling a power-supply mode transition and is used for serial communications. Lengths of change periods, during which a logical value of the indication signal is changed and with which transitions among three or more power-supply modes are associated, are preset and stored in the first control unit and the second control unit, respectively. The power-supply mode is transitioned to a specific mode according to the lengths of the change periods of the indication signal.