Variable-frequency resonance circuit and variable-frequency filter

The invention provides a variable-frequency resonance circuit in which the pass band width is unchanged or substantially unchanged even when multiple transmission bands are set. A variable-frequency resonance circuit 10 includes first and second input/output terminals P1,P2 and a resonance circuit portion rc. The resonance circuit portion rc 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.