To provide an enhanced true-to-life atmosphere enjoyed in multipoint connecting, and reduce a calculation load at a multipoint connection unit, as well.A stream synthesizing device includes an input unit which inputs at least two coded signals each including a first downmix acoustic signal and an extended signal, each of first downmix acoustic signals being obtained by coding an acoustic signal into which at least two sound signals are downmixed, and the extended signal being for obtaining the at least two sound signals out of the first downmix acoustic signal; a coded signal generating unit which generates: a second downmix acoustic signal and an extended signal based on each of coded signals inputted by the input unit, the second downmix acoustic signal being for obtaining each of the first downmix acoustic signals, and the generated extended signal being for obtaining each of the first downmix acoustic signals out of the second downmix acoustic signal; and generate a coded signal including the generated second downmix acoustic signal, the generated extended signal, and each of extended signals included in the corresponding inputted coded signal; and an output unit which outputs the generated coded signal.