PURPOSE: To prevent a chair from interfering with surrounding objects when the chair is swivelled for a customer to sit on or rise from the seat of the chair by separating a step of the chair from the seat and providing the step and seat respectively with separate elevator devices for vertical movements.

CONSTITUTION: A power section comprises an elevator device for the vertical movement of a step 1 with a master cylinder 2 and another elevator device for the vertical movement of a seat 3 with a sub-cylinder 4. When the seat 1 and the step 2 are to be lowered, they are synchronously lowered by opening a switching valve 31. The seat 3 can be moved independently vertically by operating a motor 28, opening a switching valve 30 to supply oil independently to the sub-cylinder 4, and opening a switching valve 32 to independently discharge oil. Optimum heights of the seat 3 and the step 1 can be set according to the physical build of a customer by changing their relative heights. Even if the chair is swivelled, no interference with surrounding is caused because of absence of projections out of the seat.