PURPOSE: To prevent the displacement (sinking) of an auxiliary step even in the case where persons are placed on the auxiliary step, improve the function of a driving engine and other transmission mechanism, and simplify structure.

CONSTITUTION: When a driving engine equipped with a torque limitter is used as a driving source of an auxiliary step 2, rigidity is improved by generating an overload state where an upward torque acts on the auxiliary step 2 in the stop of the projection of the auxiliary step 2, and in a swing mechanism for a step plate, each pivotal shaft of a driven arm, first connecting rod, and a link arm is set on a straight line, and the reactive force generated in the case when persons are placed on the step plate is received by a rotary shaft on the undersurface of the step, and the transmission of the reactive force to the driving shaft of the driving engine is prevented, and each of the capacity and rigidity of the driving engine is reduced to the necessary min. value.