The switching characteristics of a hitless wavelength-selective switch with double-series-coupled microring resonators depend on the coupling efficiencies between the busline waveguide and the microring resonator and between microring resonators. To improve the switching characteristics, we analyzed and optimally designed coupling efficiencies between waveguides. The switching characteristics of the optimally designed device were measured and compared with those of a nonoptimized device. In the optimally designed device, the extinction ratios of about 40 dB in the drop-port response and of 15 dB in the through-port response were successfully obtained, which demonstrates the importance of optimizing the coupling efficiencies.