PURPOSE: To secure safety by spacedly arranging plural proximity switches for detecting displacement of panels as an impedance change on region having a certain area on the reverse side of skirt guard panels on both sides of a footstep.

CONSTITUTION: Plural highly sensitive proximity switches 24 are arranged at expected intervals in a footstep passing area on the reverse side of skirt guard panels 22 on both sides of a footstep 11. The proximity switches 24 have a high frequency oscillating circuit on the inside, the impedance is changed when the panels 22 are displaced, and the detecting surface is supported on the reverse of the panels 22 so as to leave microscopic clearance. A recess is formed in an inside ledge 23 for supporting the panels 22 wherein the sensitivity adjusting part 26 for adjusting a preset value of a detecting area of the respective proximity switches 24 is arranged. Therefore, even if the panels 22 are microscopically deformed by contact of a passenger, the impedance of the circuit of the switches 24 changes, and displacement of the panels 22 is detected to raise a warning.