The present invention relates to a sliding window, including a windowsill that is fixed to a wall and is provided with at least one rail inside, and a sash having a roller structure that is in contact with the rail of the windowsill and conversion means for driving the roller structure. The conversion means causes the sash to be pressed close toward and removed from the windowsill, thereby removing a gap between the sash and the windowsill. Accordingly, not only airtight, watertight, insulation, and soundproof performance can be improved, but also wind pressure resistance performance can be improved.