A microstrip antenna for circularly polarized waves comprising a dielectric substrate (2), on the surface of which only a radiation electrode (3) for generating the circularly polarized waves is formed. A signal line (4) of a radiation electrode feeding coplanar line is formed on the back of the dielectric substrate (2), and a grounding electrode (5) is formed on the entire back of the dielectricsubstrate that avoids the formed area of the signal line (4). The signal line (4) takes a mode, in which it is formed to extend from the back end edge (7) of the dielectric substrate (2) to a midway position toward a radiation electrode center position (O) on a dielectric substrate back face (2b). The microstrip antenna can improve the circular polarization characteristics easily and can be lowered in cost and easily reduced in size.