present invention can be attained but it is preferable that the heat resistant pipe 9 is arranged at a location such that bubbles are supplied to the glass ribbon 10 where its viscosity is 10 3 to 10 6 poise in one specific example of an embodiment of the present invention a porous alumina pipe with 20 mm outer diameter was arranged traversing the glass ribbon at a position where the viscosity of the glass ribbon was 10 4 5 poise nitrogen gas was fed through this porous pipe at 50 cm 3 min and when nitrogen bubbles of 1 u03bcm to 1 mm size were generated in the molten tin tiny convex and concave portions of about 0 1 u03bcm to 2 mm diameter were formed continuously in the bottom face of the glass ribbon in this example the conveyance speed of the glass ribbon was about 7 m min and the thickness of the glass sheet was about 6 mm on the other hand the face 10 a of the glass ribbon not contacting the molten tin i e the top face had a very high surface smoothness as is the case without the bubbles one advantage of this embodiment is that the size and the shape of the convex and concave portions formed in the surface of the glass ribbon can be controlled easily in this example it is also easy to control the distribution of the unevenness by changing the flow amount of the nitrogen for example moreover the shape of the surface unevenness can be controlled with the location where bubbles are generated i e the position of the heat resistant pipe it could be verified that by controlling these conditions convex.