PURPOSE: To easily obtain belows excellent in durability and strength by interposingly pressing a cylindrical fluororesin sheet by means of a projecting roll and a recessed roll from the inside and the outside for each certain interval, and forming the ridge line portion of a projecting portion to be thinner than other portions.

CONSTITUTION: A recessed roll 14 on the outside and a projecting roll 13 on the inside are rotatingly brought close to a cylindrical fluororesin sheet S1, which is interposedly pressed by a projecting portion 13a and a recessed portion 14a to be partially rolled out. At this time, the inside of the sheet S1 is subjected to a flow phenomenon in which the surplus thickness thereof is shifted in the axial direction due to the pressing action of the projecting portion 13a while the outside being curled by the recessed portion 14a, and the sheet S1 is curved in a semicircular form due to the permanent deformation and curling, simply forming a thin-walled ridge portion. By repeating this for each defined interval, a plurality of ridge portions and valley portions are formed, thereby forming bellows expanding/contracting in the axial direction. Hence, bellows can be easily formed only by the use of rolls 13, 14.