Determination of protein charges under physiological conditions by field effect transistors (FETs) is challenging because of the screening effect by solution counter ions We showed that sugar-chain-modified FETs can detect proteins under physiological conditions Protein charges are detectable on the gate interface by applying sugar chains as receptors these sugar chains are short and highly flexible Threshold voltage shifts caused by adsorption of SSA lectin from Sambucus sieboldiana onto sialylglycan-modified FET showed a linear relationship with SSA concentrations under physiological conditions