A digital transmission system having at least a client device and a transmission device and rate-adjusting a client signal transmitted from the client device to the transmission device as necessary, to accommodate/multiplex the signal in a frame, is provided. The transmission device has a rate adjusting unit and a framer processing unit. The rate adjusting unit encapsulates the client signal using a predetermined frame structure, and inserts an idle pattern, as necessary, to perform rate adjustment to a bit rate that can be accommodated in the frame. The framer processing unit accommodates/multiplexes the signal into the frame after the rate adjustment. The digital transmission system accommodates, or accommodates and multiplexes, a bit string of the client signal directly in a payload portion, accommodates, or accommodates and multiplexes, the bit string in the payload portion after subjecting it to a reversible digital signal processing.