In a circularly polarized wave antenna apparatus including a non-reciprocal transmission line apparatus having forward and backward propagation constants different from each other, the non-reciprocal transmission line apparatus includes a transmission line part for a microwave, a series branch circuit equivalently including a capacitive element, and a shunt branch circuit branched from the transmission line part and equivalently includes an inductive element. The non-reciprocal transmission line apparatus is formed in a nonlinear shape and magnetized in a magnetization direction different from a propagation direction of the microwave. The non-reciprocal transmission line apparatus includes first and second reflectors provided at both ends of the non-reciprocal transmission line apparatus, respectively, and reflecting a signal; and satisfies that a phase difference between each pair of line parts located at positions opposed to each other across a substantially central part of the non-reciprocal transmission line apparatus is substantially 180 degrees.