To provide a small-sized and thin type circularly polarized wave antenna, an antenna design simulator for designing the antenna, and a radio module equipped with the antenna.

The antenna is constituted of a group of conductor lines 15a formed from a tabular metallic conductor 15 and a feed part 14 connected to the conductor lines 15a and is so configured that projections on two mutually orthogonal axes of a current excited in the conductor lines 15a have substantially equal absolute values and have about 90° for the absolute value for the deflection angle difference, and the reactance component of impedance of the feed part 14 becomes about 0.