A variable-frequency resonance circuit includes first and second input/output terminals and a resonance circuit portion. The resonance circuit portion includes a first inductor and first and second LC series circuits. The resonance circuit portion is connected between a ground and a transmission line that connects the first and second input/output terminals. The first LC series circuit includes a second inductor and a variable capacitor connected in series with each other. The second LC series circuit includes a third inductor and a fixed capacitor connected in series with each other. The first and second LC series circuits are connected in parallel between the first inductor and a ground. The first and second inductors are configured such that positive-coupling mutual inductance is produced therebetween.