A backrest adjusting device for office chairs comprising: a seat portion having a seat
cushion and a stand for supporting the seat
cushion; a back having a frame, a back
cushion fixed on the surface of the frame, a connecting member for connecting the bottom of the back to a rear side of the seat portion, a hollow containing space formed at the middle section of the back, an n-shaped member being disposed in the containing space and extended backward; and a first attaching device installed on the n-shaped member of the containing space, thereby achieving the adjustment of the three-dimensional space of position, longitudinal buffer and
angular rotation. Moreover, the back further includes a support member disposed at the top of the back, and a second attaching device installed on the support member and having a structure identical to the first attaching device. Meanwhile, the first attaching device has an attaching body made in compliance with the shape of a human
waist to form a backrest bump. Furthermore, the second attaching device has an attaching body made in compliance with the shape of a
human head to form a headrest. In this way, the three-dimensional space such as position, height, longitudinal buffer and
angular rotation of a backrest or a headrest is adjustable.