PURPOSE: To suppress unnecessary waves generates in a triplet strip line by forming a through hole in the vicinity of a feeder line, as well in addition to the four corners of an intersection part where a slot and the feeder line are spatially intersected.

CONSTITUTION: This planner antenna is composed of dielectric substrates 1, 4 and 7 and a ground conductor 8, a radiation element 2 is formed on the surface of the substrate 1, and a full-surface conductor 3 is formed on the surface of the substrate 4. A slot 5 is formed at a position electromagnetically coupled with the radiation element 2, and a feeder line 6 is formed on the surface of the substrate 7. Through holes 9 are added to the four corners of the slot 5 so as to conduct the conductor 3 and the ground conductor 8 and provided in the vicinity of the feeder line 6, as well. Thus, since the plural through holes 9 are provided along the feeder line 6 rather than providing the through holes so as to completely surround the slot, the unnecessary resonance frequency having the characteristic of a reflection loss is almost eliminated, and the generation of the unnecessary waves can be suppressed.