PROBLEM TO BE SOLVED: To provide a biological signal sensor system which accurately recognizes the presence of a subject and outputs a biological signal of the subject only when the subject is present, and a biological signal sensor used for the system.SOLUTION: A biological signal sensor comprises: a first electrode 12 integrally laminated on one surface of a piezoelectric sheet 11 a second electrode 13 integrally laminated on the other surface of the piezoelectric sheet 11 and a third electrode 15 integrally laminated on the second electrode 13 through a deformation layer 14 which is to be deformed so as to be elastically restorable by pressure.