A method of producing thin sheet glass by the float process, in which molten glass is formed into a thin layer on the surface of molten metal by making molten glass pass through a vertically narrow gap given by a sluice wall which extends above and transversely of the molten metal bath, and the molten glass layer is stretched to further reduce its thickness. By forming the sluice wall with a slant surface on the front side facing a pool of molten glass so as to smoothly introduce the molten glass into the gap beneath the wall and by using a practically isotropic graphite which is not greater than 23% in its porosity as the material of the sluice wall, defectless sheet glass thinner than 1 mm can be obtained. Preferably the porosity of the graphite is at least 9%. For the quality of the sheet glass, it is favorable to form the sluice wall with a vertical surface on the rear side and another slant surface which interposes between the lower edge of the slant front surface and the lower edge of the vertical rear surface to allow the molten metal beneath the wall to smoothly raise its level from a lowered level beneath the molten glass pool.