To provide a flat sash structure which is free from a fear of slipout of metal sashes during opening/closing movement thereof.

According to the flat sash structure for slidably supporting the metal sashes 1a, 1b, sash sills 2a, 2b are set up by detachably combining base members 20a, 20b each consisting of a bottom wall 201 and an erect wall 202 erected on an indoor side of the bottom wall 201, with frame members 21a, 21b formed by rollably mounting sash rollers 10 of the metal sashes 1a, 1b on an upper surface of the bottom wall 201, respectively, whereby penetration grooves 23a, 23b are defined by the frame members 21a, 21b and the erect walls 202, respectively. Further groove-like guide portions 22 opening downward are formed at one side edges of the penetration grooves 23a, 23b, respectively. Then the metal sashes 1a, 1b have guide pieces 11 protruding downward from lower edges thereof, respectively, and the guide piece 11 each have a bottom portion 12 protruding to the guide portion 22, and a tip portion 13 protruding upward from the bottom portion, which cooperatively form a hook. Thus the guide pieces 11 are inserted into the penetration grooves 23a, 23b and then the tip portions 13 are fitted into the guide portions 22, respectively.