To form a thin line with good accuracy even by applying colored rubber cement by using a particular shape of mouthpiece and a transfer roll.

Marking lines are transferred by a transfer roll 5 on the top surface of a projection 4 formed on the outer surface of an extrusion member 3 for tread rubber. The gap between the transfer roll 5 and the extrusion member 3 is appropriately selected within about 0. 75 to 2.5 mm in accordance with the height of the projection 4. The width of the marking lines are determined by the top width of the projection 4, since the transfer of the marking lines are conducted by contacting the transfer roll 5 to the top surface of the projection 4. As a result, the thinning processing of the marking lines are achieved without causing the conventional problem of clogging of a nozzle.