In this paper, we report on miniaturization of chemical sensor using Surface Plasmon Resonance (SPR). This sensor has an Au grating on a surface of an n-Si wafer. The Au grating enables excitation SPR that can be detected electrically with the device. We demonstrated a backside illumination method to excite SPR so that the excitation light was introduced from the n-Si side, instead of the Au side. The excitation light from the backside can reach the Au grating without traveling through the sample liquid so that the unwanted attenuation of the illumination light by the sample can be avoided. The excitation light is diffracted by the grating, and the diffracted light couples to free electrons on the gold surface as SPR. Since SPR excites free electrons on the Au, they are detected as photocurrent at the Schottky junction at the Au/n-Si interface. In this study, we compared the current responses by the backside illumination method with changing the refractive index of sample using air and pure water. Depending on the change of the sample, peaks appeared at different angles in the currents response curve. We verified that the proposing[GRAPHICS]device can discriminate sample species on the Au, and can perform as an SPR sensor.