PURPOSE: To obtain a microstrip array antenna with excellent electric performance, lightweight and high rigidity by providing a reinforced beam in the form of grating to a ground conductor formed with a metallic foil coated to one face of the surface of a dielectric so as to apply lining.

CONSTITUTION: The ground conductor 4 is formed by coating a metallic foil to one surface of a sandwich panel where dielectric skins 6a,6b are coated on both faces of a dielectric core 5 and a radiation element 2 and a feeding line 3 are formed by coating an optional shape of metallic foil to the other surface. The reinforced beam 7 made of a material having high specific rigidity and strength such as a carbon fiber reinforced plastic and an aramide fiber reinforced plastic is arranged in the form of grating to the outside of the ground conductor 4 for the purpose of reinforcement. Thus, the antenna is hardly bent, the deterioration in the electric characteristic due to deficiency of face accuracy is prevented, the equivalent dielectric constant of the base plate is decreased and the electric performance of the antenna is improved.