A filter for filtering electrical signals comprises a substrate and a plurality of resonators formed circularly to surround the center of the substrate. Each resonator has an arcuate part at a radially outermost part and a pair of linear parts extending from ends of the arcuate part in a radially inward direction. The arcuate part is located at the same distance from the center of the substrate. The linear parts of each resonator have different lengths from each other thereby to provide a sharp decrease of gain at ends of the frequency passband in a filtering response.