Abstract: A height adjusting device for a chair consists of an inner tube having a top end fixedly connected to a chair seat and a bottom end fixedly connecting with a bushing, a positioning tube extending upwardly from a chair base, having an upper end mounted with a sleeve for receiving the inner tube and a base plate defining a plurality of downward protrusions, a screw having a threaded upper section for threadedly engaging with the bushing and a reduced lower section defining an annular stepped edge and a bottom end extending through the base plate to fixedly attach with an engaging member defining a plurality of upward protrusions for releasably engaging with the downward protrusions of the base plate, the threaded upper section of the screw defining a top projection and a bottom projection for limiting the traveling of the bushing therealong, a spring and a bearing assembly being mounted around the reduced lower section of the screw and located between the base plate and the threaded upper section of the screw.