PURPOSE: To eliminate the change in support points of respective arms for a floor and always support stably the floor by constituting expansively the arms for lifting the floor in combination of inner and outer tubes and providing actuator for expanding and contracting the arm in each arm.

CONSTITUTION: In a lift device 20 for liftint a lift floor 3 capable of opening and cloing an opening part 2 provided in a stage floor 1 when it is applied to an elevator in a stage, two pairs of a pair of two arms 21 relatively pivotable supported by a shaft 22 under the condition of crossing in the axially approximately central parts are provided. The upper ends of the respective arms 21 are pivotably supperted 25 by the lower surface of the lift floor 3 and the lower ends are pivotably supported 28 by a bitt floor 7. The respective arms 21 are composed of outer tubes 30 in the central part and inner tubes 31 having both ends of the outer tubes fitted slidably therein, and two jacks 32 as actuators are incorporated in the arms 21 respectively. The repective jacks 32 are provided to be driven synchronously by two motors 37 through a power transmitting mechanism consisitng of a reduciton gear 34, a drive shaft 35 and a bevel gear box 36.