To provide a low-cost auxiliary step device for getting on and off a bus, which is installed directly to the lower side of the step of a bus of standard floor specifications, allows use and accommodation through simple procedure, and which does not require to equip the bus itself with anything specifically.

A configuration according to the present invention includes a pair of mounting parts 12a installed on a base 10, links 14 and 16 in pairs dangled at each side face of the mounting part 12a, a step 20 to which the bottoms of the links 14 and 16 are attached, and a pair of air cylinders 18 where the rear of the cylinder is attached to the base 10 and a support 18a etc., is attached to one of the links 16 in the condition that a step 20 of a bus is protruded and sunk outward so as to position it at the lower stage of a step 30 for getting on and off the bus.