A technique to visualize living cell activation in a real time manner without any labeling is required in the fields of life sciences and medicine. We have reported that surface plasmon resonance (SPR) sensors detect large changes of refractive index (RI) with living cells, such as mast cells, human basophils and lymphocytes. However conventional SPR sensors detect only an average change of RI with thousands of cells at detectable area on a sensor chip. Therefore, we developed an SPR imaging (SPRI) sensor with a CMOS camera and an objective lens in order to visualize RI of individual living cells and their changes upon stimuli. The SPRI sensor we developed could detect reactions of individual rat basophilic leukemia (RBL-2H3) cells and mouse keratinocyte cells in response to specific or nonspecific stimuli. Moreover, the sensor could detect the reactions of individual human basophils isolated from patients in response to antigens (allergens). Thus the technique can visualize the effect of various stimuli, inhibitors and/or conditions on cell reactions as change of intracellular RI distribution at single cell levels. Establishment of the technique to rapidly isolate cells from patient blood should enable us to utilize SPRI system as a high throughput screening system in clinical diagnosis, such as type I hypersensitivity and drug hypersensitivity, and as a tool to reveal novel phenomena in evanescent fields around plasma membranes.