PURPOSE: To improve the consistency of a slot line horn type antenna circuit used for a microwave integrated circuit (MIC) with space and to obtain a wide band and a high gain by forming slot line horn antennas in nearly the same shapes on both surfaces of a dielectric substrate.

CONSTITUTION: A conductor pattern 2A on a substrate surface IA of the dielectric substrate (MIC substrate) forms a normal slot line horn antenna. A conductor pattern 2B is provided on the other surface 1B of the substrate 1. This conductor pattern 2B constitutes the 2nd slot line horn antenna which has a horn 3B in the same shape with the 1st horn antenna (conductor pattern) 2A and a slot 4B with specific length (short in this case). One conductor 5 (while the other conductor is the conductor pattern 2) of a strip line to an MIC (not shown in a figure) fitted on the substrate surface 1B couples with the slot line 4A of the 1st slot antenna by the intervention of an electric or magnetic field. The antenna circuit of said constitution allows the 1st horn antenna 2A to vary in impedance by the 2nd horn antenna 2B, thus improving the matching with space.