Abstract: A seat back unit connected to a seat of a chair comprises a main frame having vertical and horizontal sections. A first hinge portion is disposed on a top end of the vertical section, and a resting seat with an embedded concave portion is fixed to a middle segment of the vertical section. Connection projections are disposed on a junction between the vertical and horizontal sections. A supporting member has one end formed with a second hinge portion, and the other end formed with a crossing portion embedded into the embedded concave portion. A backrest member is composed of vertical and transversal curved sections. The vertical arch section has third and fourth hinge portions corresponding to the first and second hinge portions, respectively. A herringbone-shaped structure composed of the vertical arch section and the transversal curved sections conforms to the profile variation of the human body.