A client signal mapping circuit for accommodating a client signal in a transmission frame, comprising: a buffer unit configured to temporarily store the client signal; a buffer used amount monitoring unit configured to report a used amount of the buffer unit; and an overhead insertion unit configured to add an overhead to the client signal read from the buffer unit to form a transmission frame; characterised by a stuff processing determination unit configured to determine necessity of insertion of a stuff byte into the transmission frame and a stuff processing amount based on the used amount of the buffer unit; and a reading control unit configured to perform control for reading the client signal from the buffer unit based on determination by the stuff processing determination unit, the stuff processing determination unit comprising: a comparison unit configured to compare the used amount of the buffer unit reported from the buffer used amount monitoring unit with a predetermined reference value; a pseudo random sequence generation unit configured to output a pseudo random sequence signal; and a first adding unit configured to add the pseudo random sequence signal and the comparison result output from the comparison unit, and to output stuff information indicating necessity of insertion of the stuff byte and the stuff processing amount based on the adding result of the comparison result and the pseudo random sequence signal. a buffer unit configured to temporarily store the client signal; a buffer used amount monitoring unit configured to report a used amount of the buffer unit; and an overhead insertion unit configured to add an overhead to the client signal read from the buffer unit to form a transmission frame; characterised by a stuff processing determination unit configured to determine necessity of insertion of a stuff byte into the transmission frame and a stuff processing amount based on the used amount of the buffer unit; and a reading control unit configured to perform control for reading the client signal from the buffer unit based on determination by the stuff processing determination unit, the stuff processing determination unit comprising: a comparison unit configured to compare the used amount of the buffer unit reported from the buffer used amount monitoring unit with a predetermined reference value; a pseudo random sequence generation unit configured to output a pseudo random sequence signal; and a first adding unit configured to add the pseudo random sequence signal and the comparison result output from the comparison unit, and to output stuff information indicating necessity of insertion of the stuff byte and the stuff processing amount based on the adding result of the comparison result and the pseudo random sequence signal. a comparison unit configured to compare the used amount of the buffer unit reported from the buffer used amount monitoring unit with a predetermined reference value; a pseudo random sequence generation unit configured to output a pseudo random sequence signal; and a first adding unit configured to add the pseudo random sequence signal and the comparison result output from the comparison unit, and to output stuff information indicating necessity of insertion of the stuff byte and the stuff processing amount based on the adding result of the comparison result and the pseudo random sequence signal.