A ring resonator with wider FSR is expected as an Add/Drop filter for the WDM system. In ring resonators, wider FSR is incompatible with less bending loss, because the FSR of a ring resonator is proportional to the inverse of its radius. Therefore the double-ring structure has been proposed to solve this problem. We analyzed this structure by the method based on the transfer matrix principle and signal flow chart. As a result, the FSR of more than 4THz and twenty times as wide as the one of single-ring structure is obtained. Moreover, the proper transmission characteristics are obtained for the structure of the input waveguide coupled with outer ring and output waveguide coupled with inner ring, and vice versa.