PURPOSE: To improve durability and to simplify a chair with a seat back and make the same in a compact size by connecting the horizontal side portion of a seat back supporting rod and a seat receiver to each other through a longitudinal slide mechanism comprising a pin and a circular-arc slit.

CONSTITUTION: When both front and rear pins 27, 28 are slid in slits 9, 10, 21, 22, a seat back 18 fitted to the vertical side portion 19 of a seat back supporting rod 2 is tilted forward and backward according to a reclining orbit of a person, taking a lateral virtual horizontal axis A set above a seat portion 13 and in front of the seat back 18, that is, a user's substantial loin joint axis as the central axis. A pin 30 is supported horizontally between both side plates 4, 5 of the forward end in the recess portion of a seat receiver 1, a pin 31 is disposed on the forward end portion of a horizontal side portion 20 of the back seat supporting rod 2, and a compressed gas spring 33 is fitted to both pins 30, 31. The horizontal side portion 20 of the back seat supporting rod 2 is energized backward by the action of the gas spring 33 so that the back seat 18 is always held in an erecting state.