To provide a sealing device for the meeting sections of sliding doors with which a sealant makes close contact only when the doors are fully closed.

The sliding door sealing device 1 includes: a housing 2 having parallel, vertically-elongate right and left guide plates 21 and 22 with a pair of forwardly inclined guides 21a and 22a cut therein and with a pair of forwardly inclined guides 25a disposed at their respective lower ends, the right and left guide plates being connected together at the rear faces of their respective upper portions by a connecting plate 23, with the housing also having a guide plate 27 secured to the front faces of the guide plates 21 and 22 and having a vertically-wide horizontal guide 27a and a middle, narrow horizontal guide 27b cut therein; activating members 3 which slide along the right and left guides 27a; a longitudinal shaft 4; a horizontal shaft 5a slidable against the inclined guides 21a and 22a; actuating members 8 connected together by a connection part 81 having a one-way downwardly inclined guide 81a cut in the front side of a vertically elongate, horizontal slide plate 82 having at its lower end an inclined portion 82c with the same gradient as the inclined guide 25a; and an elastic sealant 9 capable of being held in a watertight condition. The sliding door sealing device 1 and the other sliding door sealing device 1', both of which are in opposite and symmetrical relation, are attached to the meeting sections of the doors at lower corners.