A chair having an adjustable backrest for spinal protection having a seat plate part with a plurality of supporting legs capable of sitting a user thereon, a cushion part with a lumbar support (a), a thoracic support (b), and a cervical support (c) formed directly on a back region of the seat plate part and that supports and protects a spine including lumbar, thoracic, and cervical vertebras of a user at the back region thereof, and a backrest for fixing and supporting the cushion part thereto comprising: a backrest height adjusting means for elastically coupling the backrest to the seat plate part and adjusting the height of the backrest; a lumbar supporting plate formed between the cushion part and the backrest and that supports flexibly the lumbar vertebra, whose position is changeable according to the user's sitting height of the cervical vertebras and sitting height of the lumbar vertebras; a rack and pinion means connected and fixed to a fixing bracket, which is formed at an end portion of the backrest height adjusting means and that allows the backrest and lumbar supporting plate to slides vertically so as to control flexibly the height thereof; and a backrest fixing guide bracket formed at a lower end portion of the backrest so as to fix the backrest thereto and that inserts the backrest height adjusting means therein so as to slide verticallyalong the backrest height adjusting means to slidably attach the backrest fixing guide bracket to the backrest height adjusting means; characterized in that the rack and pinion means comprises a big pinion and a small pinion formed in contiguity with each other so as to be rotated to a fixing axis of the fixing bracket, the backrest comprises a backrest rack interlocked with the big pinion so as to linearly and vertically move the backrest, the lumbar supporting plate comprises a plate rack formed at an inside thereof and interlocked with the small pinion so as to linearly and vertically move the lumbar supporting plate, and the backrest comprises a slot groove for vertically sliding and guiding the fixing axis. a backrest height adjusting means for elastically coupling the backrest to the seat plate part and adjusting the height of the backrest; a lumbar supporting plate formed between the cushion part and the backrest and that supports flexibly the lumbar vertebra, whose position is changeable according to the user's sitting height of the cervical vertebras and sitting height of the lumbar vertebras; a rack and pinion means connected and fixed to a fixing bracket, which is formed at an end portion of the backrest height adjusting means and that allows the backrest and lumbar supporting plate to slides vertically so as to control flexibly the height thereof; and a backrest fixing guide bracket formed at a lower end portion of the backrest so as to fix the backrest thereto and that inserts the backrest height adjusting means therein so as to slide verticallyalong the backrest height adjusting means to slidably attach the backrest fixing guide bracket to the backrest height adjusting means; characterized in that the rack and pinion means comprises a big pinion and a small pinion formed in contiguity with each other so as to be rotated to a fixing axis of the fixing bracket, the backrest comprises a backrest rack interlocked with the big pinion so as to linearly and vertically move the backrest, the lumbar supporting plate comprises a plate rack formed at an inside thereof and interlocked with the small pinion so as to linearly and vertically move the lumbar supporting plate, and the backrest comprises a slot groove for vertically sliding and guiding the fixing axis.