The invention relates to the technology of a chair whose backrest can be automatically moved and adjusted. The chair of the present invention includes a seat cushion, a backrest, and a support leg, and is composed of an upper plate, a middle plate, and a lower plate, which face up and down at a predetermined interval while being kept disposed between the seat cushion and the support leg. Arrangement; a hydraulic booster composed of a hydraulic booster cylinder, wherein the hydraulic booster cylinder is arranged between the middle plate and the lower plate, and performs elastic buffering action against the force from the upper and lower directions, that is, when the user sits on the seat cushion When going up, based on the user's weight, the middle plate is elastically compressed, and the hydraulic release is delivered to the reciprocating action cylinder set between the upper plate and the middle plate. state, and absorb the hydraulic pressure delivered to the above-mentioned cylinder; the guide box, which is located between the upper plate and the middle plate, is fixed on the top of the middle plate facing the front-rear direction; the reciprocating action cylinder is horizontally arranged inside the guide box, when When the hydraulic booster cylinder of the hydraulic booster releases the hydraulic pressure, the piston rod moves forward with the inflow of the hydraulic pressure. When the hydraulic pressure is recovered from the hydraulic booster cylinder, the piston rod retreats with the release of the hydraulic pressure; The formed backrest moving part is installed horizontally through the moving long holes on the left and right sides of the guide box, and passes through the guide box in a vertical cross direction. At the same time, it is connected to the front end of the piston rod of the reciprocating cylinder. The forward and backward movements of the guide box are carried out within the moving range of the moving long hole of the guide box; the backrest frame, which is connected to both ends of the main joystick of the backrest moving part, performs forward or backward actions together with the main joystick; The sub joystick is movably provided at the rear between the upper board and the middle board, and both ends thereof perform forward or backward motions together with the backrest frame in a state of being connected to the backrest frame.