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Resonator eigenmodes in symmetric quasi-stadium laser diodes consisting of two curved end-mirrors and two straight side-wall mirrors are theoretically studied. We obtained the resonator eigenmodes under typical resonator conditions, namely, a concentric and a near-concentric resonator conditions, a confocal resonator condition, and a geometrically unstable resonator condition. We used the extended Fox-Li mode calculation method which includes the effect of the total internal reflection at the side-wall mirrors. Unique resonator eigenmodes are obtained. Under the concentric and the near-concentric resonator conditions, not only the Hermite-Gaussian modes but also ring-trajectory modes are obtained. Under the unstable resonator condition, the diffraction loss of the resonator eigenmode significantly depends on the wavelength. At the wavelength of minimum diffraction loss, the highly directional beams propagate along the closed repetitive trajectories inside the resonator.