a tunable filter has a plurality of variable capacitors and a plurality of inductor elements , each being formed on a common substrate , a filter circuit formed by using at least a portion of the plurality of variable capacitors and a portion of the plurality of inductor elements , a monitor circuit formed by using at least a portion of the plurality of variable capacitors and a portion of the plurality of inductor elements , a detecting circuit which detects a prescribed circuit constant of the monitor circuit , a storage which stores information relating to a reference circuit constant of the monitor circuit , and a capacitance control circuit which controls capacitance of the variable capacitors in the monitor circuit and capacitance of the variable capacitors in the filter circuit , based on a result detected by the detecting circuit and information stored in the storage .