To impart elasticity to a filter medium pack without detriment to its rigidity.

A filter medium pack 40 is disposed in the inside of a square filter frame. The filter medium pack 40 has many folds 41 formed by a folding processing. The many folds 41 are arranged in the horizontal direction H which rectangularly crosses the folding direction. A plurality of linear beads 45 are formed on each of the face 40F and back 40B of the filter medium pack 40. The linear beads 45 extend in the horizontal direction H while they join the tops 41T of the folds 41 together. No filter medium beads 40 are formed at the left and right side parts 40L and 40R of the filter medium pack 40, and therefore the folds 41 at these side parts are not joined together with linear beads 45. As the results, the elasticity at the lateral directions L of both side parts 40L and 40R is higher than that at the remaining part. Thus, the elastic forces of both side parts 40L and 40R secure sealability between the end face of each of the side parts 40L and 40R and the inside surface of a filter frame.