A planar dielectric integrated circuit is provided such that energy conversion loss between a planar dielectric line and electronic components is small and that impedance matching between them can be easily obtained. By providing slots which oppose both main surfaces of a circuit substrate, two planar dielectric lines are constructed. A slot line, and a first line-conversion conductor pattern which is connected to the electromagnetic field of the slot line and a first planar dielectric line in order to perform line conversion, are provided at the end portion of the first planar dielectric line, including a slot. A coplanar line and a second line-conversion conductor which is made to project in a direction at right angles to a second planar dielectric line is provided at the end portion of the second planar dielectric line, including a slot. A semiconductor device is placed in such a manner as to be extended over the coplanar line and the slot line.