A semiconductor integrated circuit device with a balun which is formed above a conductive semiconductor substrate and which includes a dielectric film, an unbalanced line for transmitting an unbalanced signal, and balanced lines for transmitting a balanced signal. The unbalanced line is placed opposite to the balanced lines via a nano-composite film that is a region of the dielectric film. The nano-composite film, interposed between the unbalanced line and the balanced lines, has a relative permittivity higher than that of other regions of the dielectric film. This allows suppression of electromagnetic coupling of transmission lines or passive elements other than the balun, thereby providing a semiconductor device with a wide-band and small-size balun.