A linear sensor 21 includes an attachment base 22 attached to a metallic edge of a slide door 2 capable of opening and closing a platform 4, a skin cover part 23 which bulges from the attachment base 22 and has a hollow part 24 in the inside, an insulator 25 which has flexibility and also is embedded in the hollow part 24, and a plate-shaped electrode 26 opposed to the attachment base 22, at least a portion of the electrode being embedded in the insulator 25. In the hollow part 24, a gap 27 is disposed between the electrode 26 and the attachment base 22 and also, the electrode 26 is set in a plus pole and the attachment base 22 is set in a minus pole as the grounding, and proximity and/or contact between a foreign body and the skin cover part 23 is detected based on a change in capacitance between the electrode 26 and the grounding minus pole.