The present invention provides a manufacturing method of a glass substrate for an image display device having high picture quality. The reducing force in a float furnace is controlled to be decreased so that Sn++ content on a surface of the glass substrate forming an Ag electrode is a predetermined value or less. When the resultant Sn++ content on the surface of the glass substrate forming the Ag electrode exceeds the predetermined value, the surface is partially removed to decrease the Sn++ content to the predetermined value or less to suppress the occurrence of yellowing of the glass substrate.