We discuss quantum dynamics in the ring systems with double Y-junctions in which two arms have same length. The node of a Y-junction can be parametrized by U(3). Considering mathematically permitted junction conditions seriously, we formulate such systems by scattering matrices. We show that the symmetric ring systems, which consist of two nodes with the same parameters under the reflection symmetry, have remarkable aspects that there exist localized states inevitably, and resonant perfect transmission occurs when the wavenumber of an incoming wave coincides with that of the localized states, for any parameters of the nodes except for the extremal cases in which the absolute values of components of scattering matrices take $1$. We also investigate the magnetic disturbance to the symmetric ring systems.
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