PROBLEM TO BE SOLVED: To provide a biosignal sensor capable of highly accurately measuring a feeble biosignal such as a pulse wave, being carriageable, and capable of easily measuring a biosignal at a visit place.SOLUTION: The biosignal sensor for detecting a biosignal of a subject includes: a piezoelectric sensor 1 an expanded sheet 5 that is integrally superposed on one surface of the piezoelectric sensor 1 and a shape-retaining plate 7 integrally superposed on the other surface of the piezoelectric sensor 1. The expanded sheet 5 is characterized in that, in a stress-strain curve, an absolute value of a variation of a stress under a compressibility of 0-10% in a compression process is ten times or more of an absolute value of a variation of a stress under a compressibility of 20-30%, and the stress under the compressibility of 20% is 1-20 kPa.
