PURPOSE: To compose a device without being limited by a stopper body, and install it even on a flat installation place by eliminating downward move of the stopper body when a seat is moved down.

CONSTITUTION: A pair of a front and a back parallel links 5 are pivotally supported on a fixed base 3 to be capable of oscillation from a forward and upward directed condition to a backward and upward directed condition, a seat 9 is provided on a support base 7 connected to the front and the back parallel links 5 to be changeable between a front-directed attitude and a back- directed attitude, an energization means 10 to energize upper ends of the parallel links 5 upward is provided, and a height limiting means 11 to regulate upward oscillation of the parallel links 5 is provided. The height limiting means 11 comprises an engagement body 12 close to a lower end of the parallel links 5 to be integrally oscillated with it, a stopper body 13 supported by the fixed base 3 to be capable of oscillation about a lateral axis, and engaged with the engagement body 12 to regulate upward oscillation of the parallel links 5, and an operation member 15 to oscillate this stopper body 13 to be engaged with/ detached from the engagement body 12.