Abstract: A backrest adjusting structure including a backrest having a retractable shaft transversely disposed near a bottom side thereof and a stop adapted for stopping the retractable shaft from rotary motion, the retractable shaft having two fixed gears at two opposite ends; two guide tubes disposed in parallel and spaced from the backrest at two opposite sides, each of the guide tubes having a longitudinal series of locating holes; fasteners adapted for securing the guide tubes in place; and two sliding sleeves respectively and slidably mounted around the guide tubes, each of the sliding sleeves having an internally toothed socket raised from the periphery and engaged with one of the fixed gears, and a radial through hole adapted for connecting to one of the locating holes of the corresponding guide tube, a locating rod balanced on an upright lug and forced by a spring into engagement with the radial through hole and one of the locating holes of the corresponding guide tube, and a press button controlled to release