Sewing device for sewing first and second strips of sealing materials with respective first and second base materials. A first guide through-hole, through which the first strip of sealing material passes, is formed in a presser foot of the sewing device adjacent to a needle hole formed in the latter. A second guide though-hole, through which the second strip of sealing material passes, is formed in a feed dog member of the sewing device at a point adjacent to that needle hole. In operation, while the first and second base materials are fed by and between the presser foot and feed dog member, the first and second strips of sealing materials are simultaneously fed together with the first and second base materials, while being precisely guided by the respective first and second guide through-holes for sewing with the respective first and second base materials.