A signal selecting device, comprising: two input/output ports (111, 112); a plurality of resonating parts (120 1 to 120 N) having a ring conductor (121 n) having a length equal to one wavelength at a resonant frequency or an integral multiple thereof and a plurality of switches (122 n -1 to 122 n -M) each of which is connected to a different part of said ring conductor at one end and to a ground conductor at the other end; a plurality of impedance transforming parts (130 0,1 to 130 N,N+1) that adjusts an impedance; and a controlling part that controls the state of said switches, said resonating parts are disposed in series between said two input/output ports, characterized in that said impedance transforming parts are disposed between said input/output ports in such a manner that the impedance transforming parts (130 0,1, 130 N,N+1) at the both ends are disposed between the input/output port and the resonating part and the remaining impedance transforming parts (130 1,2 to 130 N-1,N) are disposed between the resonating parts, N is number of said resonating parts, n is an integer from 1 to N, M is number of said switches, the fractional bandwidth (w) and the element values (g 0 to g N+1) of the original low pass filter are determined from the bandwidth and the in-band and out-band characteristics of the signal selecting device to be achieved, the admittance parameter (J 0,1 to J N,N+1) are calculated from the characteristics of the circuit of the impedance transforming part, and said switches to be turned on are selected among the switches (122 n-1 to 122 n -M) so that the susceptance slope parameters (b 1 to b N) satisfy { J 0, 1 = Gb 1 ⁢ w g 0 ⁢ g 1 J k - 1, k = w ⁢ b k - 1 ⁢ b k g k - 1 ⁢ g k J N, N + 1 = Gb N ⁢ w g N ⁢ g N + 1, where G denotes the port admittance, and k is an integer from 2 to N. two input/output ports (111, 112); a plurality of resonating parts (120 1 to 120 N) having a ring conductor (121 n) having a length equal to one wavelength at a resonant frequency or an integral multiple thereof and a plurality of switches (122 n -1 to 122 n -M) each of which is connected to a different part of said ring conductor at one end and to a ground conductor at the other end; a plurality of impedance transforming parts (130 0,1 to 130 N,N+1) that adjusts an impedance; and a controlling part that controls the state of said switches, said resonating parts are disposed in series between said two input/output ports, characterized in that said impedance transforming parts are disposed between said input/output ports in such a manner that the impedance transforming parts (130 0,1, 130 N,N+1) at the both ends are disposed between the input/output port and the resonating part and the remaining impedance transforming parts (130 1,2 to 130 N-1,N) are disposed between the resonating parts, N is number of said resonating parts, n is an integer from 1 to N, M is number of said switches, the fractional bandwidth (w) and the element values (g 0 to g N+1) of the original low pass filter are determined from the bandwidth and the in-band and out-band characteristics of the signal selecting device to be achieved, the admittance parameter (J 0,1 to J N,N+1) are calculated from the characteristics of the circuit of the impedance transforming part, and said switches to be turned on are selected among the switches (122 n-1 to 122 n -M) so that the susceptance slope parameters (b 1 to b N) satisfy { J 0, 1 = Gb 1 ⁢ w g 0 ⁢ g 1 J k - 1, k = w ⁢ b k - 1 ⁢ b k g k - 1 ⁢ g k J N, N + 1 = Gb N ⁢ w g N ⁢ g N + 1, where G denotes the port admittance, and k is an integer from 2 to N.