An optical transport system comprising: a time-multiplexing processing unit that time-multiplexes a plurality of client signals and accommodates the client signals in any of a plurality of time slots included in one transport frame; a time slot control unit that determines, when the time-multiplexing processing unit time-multiplexes the plurality of client signals, which of the plurality of time slots each of the client signals is to be accommodated in; a transport frame generation unit that divides the transport frame in which the plurality of client signals are accommodated into a plurality of transmission signals; a plurality of subcarrier transmission units (34-1 to 34-i) that are provided for the transmission signals and convert the plurality of transmission signals which are electric signals into optical signals using different optical carriers, and transmit the converted optical signals; a plurality of subcarrier reception units (41-1 to 41-i) that are provided corresponding to the plurality of subcarrier transmission units and receive the optical signals transmitted from the corresponding subcarrier transmission units, and convert the optical signals into reception signals; a transport frame termination unit that combines the reception signals converted by the plurality of subcarrier reception units to restore the transport frame; a time-demultiplexing processing unit that time-demultiplexes the transport frame restored by the transport frame termination unit to be separated into the plurality of client signals; and a power source control unit (36, 46) that controls supply of electric power to the plurality of subcarrier transmission units and the plurality of subcarrier reception units, wherein the optical transport system is characterized in that when a value obtained by calculating (the number of used subcarriers) - ((the number of time slots to which the plurality of client signals are allocated) / (the number of time slots per subcarrier)) is equal to or greater than 1, the time slot control unit determines a new allocation that accommodates the plurality of client signals sequentially from a time slot corresponding to any one of a plurality of subcarriers without proving an empty time slot, notifies the time-multiplexing processing unit and the time-demultiplexing processing unit of the new allocation, and causes the power source control unit to stop the supply of electric power to a subcarrier transmission unit and a subcarrier reception unit that transmit and receive an optical signal to which the client signals are not allocated. a time-multiplexing processing unit that time-multiplexes a plurality of client signals and accommodates the client signals in any of a plurality of time slots included in one transport frame; a time slot control unit that determines, when the time-multiplexing processing unit time-multiplexes the plurality of client signals, which of the plurality of time slots each of the client signals is to be accommodated in; a transport frame generation unit that divides the transport frame in which the plurality of client signals are accommodated into a plurality of transmission signals; a plurality of subcarrier transmission units (34-1 to 34-i) that are provided for the transmission signals and convert the plurality of transmission signals which are electric signals into optical signals using different optical carriers, and transmit the converted optical signals; a plurality of subcarrier reception units (41-1 to 41-i) that are provided corresponding to the plurality of subcarrier transmission units and receive the optical signals transmitted from the corresponding subcarrier transmission units, and convert the optical signals into reception signals; a transport frame termination unit that combines the reception signals converted by the plurality of subcarrier reception units to restore the transport frame; a time-demultiplexing processing unit that time-demultiplexes the transport frame restored by the transport frame termination unit to be separated into the plurality of client signals; and a power source control unit (36, 46) that controls supply of electric power to the plurality of subcarrier transmission units and the plurality of subcarrier reception units, wherein the optical transport system is characterized in that when a value obtained by calculating (the number of used subcarriers) - ((the number of time slots to which the plurality of client signals are allocated) / (the number of time slots per subcarrier)) is equal to or greater than 1, the time slot control unit determines a new allocation that accommodates the plurality of client signals sequentially from a time slot corresponding to any one of a plurality of subcarriers without proving an empty time slot, notifies the time-multiplexing processing unit and the time-demultiplexing processing unit of the new allocation, and causes the power source control unit to stop the supply of electric power to a subcarrier transmission unit and a subcarrier reception unit that transmit and receive an optical signal to which the client signals are not allocated.