The present invention dissolution step for dissolving the glass raw materials and, supplying a fining step, a fining the molten glass refining the dissolved glass continuously over the molten metal (9) in the float bath (2), the molten metal (9) of glass from above a molding process for molding the ribbon 5 and the lift-out take-off roll 7 glass ribbon (5) by a from the float bath (2), while conveying the glass ribbon (5) by a layer roll 8 glass of having a slow cooling step of slow-cooling to the distortion point temperature or lower, slow cooling step is a bottom of the glass ribbon (5) the glass ribbon (5), by using the injector (20, 30) provided to a lower portion of the between-layer roll 8 by supplying a gas containing a halogen element to the surface (5a), it relates to a float glass manufacturing method comprising the impurity removing process for removing foreign substances adhered to the bottom surface (5a).