Heat-assisted magnetic recording (HAMR) is a promising technology for achieving more than 10 Tbit/inch2 recording density. A near-field transducer (NFT), which forms a small light spot on a recording medium, is necessary in HAMR. However, the heat generated by the NFT would melt the NFT itself. To solve this problem, the authors have proposed a novel device, in which a metal nano-antenna is attached to a semiconductor ring resonator. In this paper, the near-field light generated by the semiconductor ring resonator with the metal nano-antenna was analyzed through a numerical simulation to optimize the structure of the device. The simulation was conducted using the finite element method based on a 3-dimensional model. It was found that how to excite a desired eigenmode selectively among some eigenmodes is important to make the device effective, and that various design parameters such as the length of the nano-antenna and the distance between the ring resonator and the nano-antenna can be optimized.