An ac surface photo voltage technique was successfully applied to construct a highly sensitive immuno-sensor by reducing the ion screening effect. A low noise amplification was attained by using a phase sensitive amplification technique, which made it possible to use low-ion density electrolyte leading to suppress the ion screening effect. The response voltage attained was as high as 7 mV for the detection of 10−8 M IgG antigen with a signal-to-noise ratio of 104.