PURPOSE: To optimatize the selectivity of a receiver by constituting an intermediate frequency filter which decides the selectivity of the receiver of an active filter or the like, and changing a filtering characteristic.

CONSTITUTION: In a heterodyne type receiver used for a mobile communication equipped with at least one frequency modulator 3.3, intermediate frequency filter 3.4, and detector 3.6, an interference detector 3.9 or a level detector is provided at the output of the intermediate frequency filter 3.4. Then, the interference amounts of an unnecessary arriving wave against a desired value is detected by an interference detector 3.9, and when a large amounts of interference amounts are detected, the classification and order of the intermediate frequency filter 3.4 is selected by a switcher 3.8 so that the attenuation gradient of the filter 3.4 can be sharp in order to increase the selectivity of the receiver. And also, when the intermediate frequency filter 3.4 is constituted of the active element, the power consumption of non-selected filters can be reduced by activating only the selected filter by the switcher 3.8.