Abstract: For accurately detecting the position of an edge of a glass ribbon in a process for manufacturing float glass, wherein the glass ribbon is moved forwardly along a longitudinal bath of molten metal, a radiation receiving edge of a radiation pyrometer is inserted transversely of the glass ribbon and thereabove from the side wall of the tank supporting the molten metal bath. The radiation receiving edge is reciprocably moved so as to detect the position at which the indication of a predetermined temperature changes, or the position where the temperature increases rapidly, and the position of the edge of the glass ribbon is detected by measuring such position of the radiation receiving edge where the temperature indicated increases rapidly.