In a method for designing an envelope detecting circuit for a video signal processing integrated circuit (IC), the number of peripheral components of the video signal processing IC is minimized by incorporating, into the video signal processing IC, a circuit device for detecting the envelope of an FM video signal and for controlling the envelope level. According to the IC designing method, the number of components can be reduced by designing all circuit elements constituting a circuit which detects the envelope of the FM video signal and which controls envelope level so that they are incorporated into the video signal processing IC at the time of designing the video signal processing IC. As a result, cost is reduced and, since mode control signal lines are reduced, the corresponding terminals of the microprocessor can be used for another purpose, thereby increasing the operating efficiency of the microprocessor.