To provide a gap closing device which is intended for a sliding door equipped with a sash roller, and which can close a gap throughout the breadth of the sliding door.

This gap closing device includes: an outer case 4 which is fitted into and supported in a groove formed on the lower end surface of the sliding door 2, which forms an approximately U-shaped cross section opened on a downside, and which has a length dimension equivalent to the breadth of the sliding door; an inner case 5 which is fitted into the outer case 4, which forms an approximately H-shaped cross section, which has a length dimension equivalent to the breadth of the sliding door, and which has tight materials 8 provided at lower ends of both sidewalls, respectively; a sliding plate 6 which is held on the internal top surface of the outer case 4 in such a manner as to be slidable along the longitudinal direction of the member; a working pin 7 which is connected to the sliding plate 6, and the leading end of which is urged outward and held in the state of being protruded with respect to the side surface of the sliding door; and the sash rollers 3 and 3 which are arranged at both the ends in the outer case 4, respectively. When the working pin 7 recedes along with the closing operation of the sliding door 2, the inner case 5 is lowered to close the gap between a lower end of the sliding door and a floor surface. The inner case 5 is lifted along with the opening operation of the sliding door 2.