To simply and accurately measure a gap through which a card passes, and to objectively and accurately adjust the above gap without depending upon worker's experience and intuition.

A pressure measuring film 36 is passed through a gap in a state where the pressure measuring film 36 is put upon an elastic mat 34 whose thickness becomes gradually thinner toward the inserting direction of a card, and the dimension of the gap is obtained from a position where the pressure measuring film 36 begins coloring. The elastic mat 34 whose thickness varies stairs-shapedly can be used. In the elastic mat 34, a plurality of projecting parts parallel to a card inserting direction is stripedly provided on a plane on at least one side to facilitate compressive deformation, and it is desired that the upper face of the projecting part is formed into a stairs-shape.