An acoustic structure of seat back, with an acoustic unit provided in a hole of padding of the seat back. The acoustic unit includes speaker unit (s) and a support box in which the speaker unit (s) is/are supportively received. The support box is accommodated in the afore-said hole of padding, with an opened side thereof facing to a forward side of the padding. A sound-conductive cushion element is supported by that support box so as to lie on a forward side of the speaker unit(s). The support box may include an acoustic hollow portion therein. Those padding and sound-conductive cushion element are covered with a trim cover assembly. A speaker support cushiony element may be provided in the support box to support the speaker unit(s), with a high sound conductivity. The speaker support cushiony element and/or the sound-conductive cushion element may be of network structure.