PURPOSE: To facilitate cushion adjustment by pivoting a swaying body on one of parallel arms beneath a seat, providing a spring between the swaying body and the other arm, and adjusting the tension of the spring.

CONSTITUTION: A pair of front and rear dual arms 8, 10 are provided between a seat frame 4 and a cylinder stand made in one body with lifting cylinder 2 provided on a base cylinder. A support body 12 is fixed to the pivotal axis 6 of one dual arm 8 and a swaying body 13 is pivoted on it. A tension spring 16 is provided between one end of the swaying body 13 and the pivotal axis 7 of the other dual arm 10. A regulator 15 is provided on the support body 12 and one end of the regulator 15 is connected to the swaying body 13. By operating said regulator 15, the swaying body 14 is moved to adjust the tension force of the tension spring 16, thereby adjusting the cushion.