A plurality of flat-plate filter elements are placed on a pallet having a plurality of holding holes. Each of the filter elements includes a rear-principal-surface electrode pattern having a ground electrode provided on a rear principal surface and a front-principal-surface electrode pattern having a principal-surface electrode provided on a front principal surface. One of a first side surface and a second side surface of each of the filter elements is placed on a printing surface of the pallet. A first side-surface electrode pattern of a point-symmetric form in the side surfaces is formed on the side surfaces placed on the printing surface of the pallet. Then, the filter elements are vertically inverted, and a second side-surface electrode pattern having the same form as the first side-surface electrode pattern is formed on the side surfaces facing each other.