An organic molecular sensor with a plasmon antenna was developed. A coaxial Ag grating antenna was designed by a finite differential time domain (FDTD) calculation and fabricated by a lift-off process. When pumping light was irradiated onto the sensor, a surface plasmon wave was generated and focused at the center of the antenna; this wave produced a strong electric field. By coating a cystamine self-assembled monolayer (SAM) onto the sensor, strong Raman scattering light produced by a surface-enhanced Raman scattering (SERS) effect was observed at the center of the device. The Raman spectrum showed good agreement with that of cystamine on a Ag surface. The Raman intensity at the center of the device increased significantly and was more than 2,000 times that outside the device. Our optical sensor can detect an unknown single molecule and can be incorporated into other sensors such as gas sensors and biosensors.