PURPOSE: To provide the method for efficiently working the edge of the mat without generating troubles, such as flow marks.

CONSTITUTION: A die 2 which is inclined at 100 to 170° with the press surface of the mat 1 at least at one end of the surface in contact with the mat 1 and consists of a plane part 2b in the parts exclusive of this inclined part 2a is used. A part of the edge of the mat 1 is heated and pressed with this die 2, by which an inclined thin part 3c and a plane thin part 3a are formed at the edge; thereafter, the die 2 is shifted in the longitudinal direction of the edge and the process of executing the similar heating and pressing in the state of positioning the plane part 2b of the die 2 in at least the inclined thin part 3c is repetitively executed plural times.