To obtain a half-wavelength side coupling filter, the center frequency of which can be adjusted even when the actual value of the center frequency deviates from a designed value and can meet specified characteristics without remaking the filter.

The printed board of a filter has a ground surface on its backside and the surface of the board is constituted by using conductive microstrip lines. The filter is provided with a heating device 103 for increasing the dielectric constant of the above-mentioned printed board and lowering the center frequency of the filter by raising the temperature of the board and a cooling device 102 for lowering the dielectric constant of the printed board and raising the center frequency of the filter by cooling the printed board.