A signal multiplexing apparatus has an OPU/ODU processing unit 4 putting a local protocol signal into an OPU/ODU signal, an ODTU processing unit 5 generating an ODTU signal from the OPU/ODU signal, a read-out position detecting unit 3 detecting a read-out position at the time that the OPU-ODU processing unit 4 reads out the local protocol signal, and a stuff allocation controlling unit 6 allocating a stuff control amount to be used when the OPU/ODU processing unit 4 puts the local protocol signal into the OPU/ODU signal and a stuff control amount to be used when the ODTU processing unit 5 generates the ODTU signal, on the basis of the read-out position detected by the read-out position detecting unit 3. The signal multiplexing apparatus can effectively absorb an amount of fluctuation in bit rate when asynchronously multiplexing the local protocol signal such as a 10GE signal in an optical transport network.