Abstract: A height adjusting device for a bicycle includes a mounting tube, a seat post, and a ratchet member. The mounting tube is configured to be inserted into a seat tube of the bicycle, and defines therein an inner tube space. The seat post is telescopically received in the inner tubular space through an upper open end of the mounting tube, and has a rack region with a plurality of rack teeth. The ratchet member has a ratchet end which is rotatable about a ratchet axis among a forward position, a normal position, and a rearward position, and which is coupled to a selected one of rack teeth so as to permit said selected one of the rack teeth to be kept engaged relative to the ratchet end.