To reduce cost by decreasing the number of members in constituting a seat so as to make it freely slidable.

This chair is manufactured by integrally molding the seat plate and its support, each with a synthetic resin. The seat support is formed with a front and rear fixed bars 9, 10 having an upward recess, while the seat plate is formed with a movable bar 13 having a downward projection; the seat plate 4 is made slidable back and forth by engaging the fixed bars 9, 10 with the movable one 13. The downward projection 13a of the movable bar 13 is fitted, so as to make it freely slidable, to a guide pin 11 put through the front and rear fixed bars 9, 10; the movable bar 13, fixed bars 9, 10 and the sliding are guided by the guide pin 11 with a spring attached, and also the seat plate is attached to the seat support in the manner in which the seat plate can not fall out from the support.