A high frequency semiconductor device includes a ground plate, an insulating layer, a power-supply conductor, an insulating interlayer, and a strip line as a line conductor. The power-supply conductor is disposed above the ground plate, with the insulating layer provided therebetween. The ground plate and the power-supply conductor have a capacitance formed therebetween. Thus, the line conductor regards the power-supply conductor as having a potential identical to that of the ground plate. This makes it possible to lay out the line conductor without considering the arrangement of the power-supply conductor. In other words, by two-dimensionally overlapping a microstrip line and a power-supply conductor in an MMIC, the degree of freedom in the device layout can be increased.