A seaming dimension adjusting device in a double seaming apparatus for double seaming a can by successive action of a first seaming roller and a second seaming roller, said seaming apparatus including a seaming chuck, a seaming roller shaft, first and second seaming rollers mounted on a lower end portion of said seaming roller shaft and a double seaming cam which actuates an upper end portion of said seaming roller shaft for shifting said first seaming roller and said second seaming roller either toward or away from said seaming chuck, wherein said seaming dimension adjusting device comprises a double seaming cam having a profile face, including a first seaming cam having a maximum displacement section of said first seaming roller and a second seaming cam having a maximum displacement section of said second seaming roller, at least one of said first seaming cam and said second seaming cam has a seaming cam adjusting means for adjusting the displacement of the maximum displacement section of the seaming roller in the seaming cam radial direction, and the shifting direction at a boundary portion between said first and second seaming cams is nearly tangential to the profile face of the cam.