The excitation mechanism of surface plasmon polaritons (SPPs) in a surface plasmon sensor with a one-dimensional (1D) Au diffraction grating on a glass substrate is studied herein. The sensitivity of the sensor for application to a refractometer is also characterized. The SPPs are excited at the following two types of interface: one between the Au grating and the glass substrate and the other between the Au grating and the medium. The simulation data for the transmittance spectra and the transmittance mapping are consistent with the experimental data even when the refractive index of the solution medium is 1.700. Therefore, the excitation mechanism of the SPPs in a surface plasmon sensor is capable of detecting the medium (n = 1.700), in which the sensor is used and clarified.