A sensor capable of detecting detection targets that are necessary to be detected with high sensitivity is provided.It comprises a field-effect transistor 1A having a substrate 2, a source electrode 4 and a drain electrode 5 provided on said substrate 2, and a channel 6 forming a current path between said source electrode 4 and said drain electrode 5; wherein said field-effect transistor 1A comprises: an interaction-sensing gate 9 for immobilizing thereon a specific substance 10 that is capable of selectively interacting with the detection targets; and a gate 7 applied a voltage thereto so as to detect the interaction by the change of the characteristic of said field-effect transistor 1A.