To have a base plate and seam lid having an improved system in order to sufficiently and simultaneously fix the base plate to a canister by providing the base plate to be mutually fitted to the seam lid with an external gasket holding groove.

The groove 64 is formed by arranging the peripheral ring 47 of the seam lid 45 on the seat face 54a of the base plate 34. The groove 64 has an external wall provided by an upright wall 52, a base part provided by a peripheral ring 47 and a groove internal wall provided by an external cylindrical wall of a rim 44. The groove 64 is formed by using the cylindrical wall 48 of the base plate 34 in combination with the seam rid 45, by which the simpler stamping of a steel sheet of the seam lid 45 for the external groove 64 is made possible. The forming necessary for provision in the groove 64 and the intricate forming of the steel sheet are lessened and the rupture strength of a filter 10 is enhanced and the production cost is reduced.