Monolithic semiconductor integrated circuit device

An integrated circuit device comprises a substrate (1); circuit elements including an active element (7) and a bias line (61 or 62) for applying a DC bias voltage to the active element (7), disposed on the substrate (1); a thermoplastic material layer (8) disposed on a region of the substrate (1); and a magnetic substance layer (9) disposed on a region of the substrate (1) including a required region on the bias line (61 or 62), and adhered to and supported by the thermoplastic material layer (8). In this structure, the magnetic substance layer (9) can be formed in an appropriate shape and at an appropriate position on the bias line (61 or 62) according to the oscillation characteristics of the active element (7), such as a transistor, and the magnetic substance layer (9) absorbs the frequency components of the oscillation of the active element (7), whereby the oscillation of the active element (7) is easily prevented. Further, since the magnetic substance layer (9) is disposed on the bias line (61 or 62), the unwanted increase in the chip area of the integrated circuit device due to the use of the prior art oscillation preventing circuit is avoided.