A sampling unit having a first receiving part (16) and a second receiving part (18), which receive a sample solution and are disposed separately from each other, wherein the first receiving part (16) comprises an identification substance (22) that binds to a substance to be detected, and separates the substance to be detected from substances not to be detected in the sample solution, and the second receiving part (18) is connected with a reference electrode (21) via a salt bridge part (25).