A gold (Au) line and space (L/S) grating antenna for the silicon-on-insulator MOS (SOI-MOS) photodiode is investigated. The SOI-MOS photodiode has the advantages such as high-speed operation, large voltage gain per electric charge, and small dark current. However, its light sensitivity is usually very low due to the small volume of light absorption. In order to enhance the light sensitivity, a surface plasmon (SP) antenna is adopted. The SP antenna is constructed with a periodic metallic structure, and can convert the incident light to optical near-field around the surfaces of the antenna. By replacing the MOS gate with the SP antenna, it becomes possible to enhance the light absorption while keeping the advantages of SOI-MOS photodiode. This paper focuses on the visible light absorption in the SOI-MOS photodiode with the SP antenna. The structural conditions for high absorption efficiency and single-peaked spectroscopic characteristics, polarized light response, and resonance mechanisms are clarified by electromagnetic simulations using the finite-difference time-domain (FDTD) method.