PURPOSE: To prevent undue load of a drive shaft by arranging a cylinder unit receiving the pressure of pressure medium element in a push rod.

CONSTITUTION: Push rods 22, 26 are guided in a housing 23 movably in the axial direction of a car body 2 and a cylinder unit 24 is disposed between the end pieces 22, 26 of the push rods 22, 26. The cylinder unit 24 is fed with pressure from an elevating/lowering bellows 1 through a pressure medium duct 3 and adjusts the length of the push rods 22, 26 in relation to the pressure. Consequently, axial movement of one end piece 22 actuates axial movement of the other end piece 26 in the opposite direction depending on the pressure of the elevating/lowering bellows 1. The one end piece 22 is preloaded against signal bodies 34, 36.