To provide a large deformation sensor which is inexpensive and compact to be easy to use and which can detect extending deformation and contracting deformation of several tens percent or larger, and to provide a seated person behavior detection sensor/monitor and a game/exercising apparatus which use the large deformation sensor and which are suitable for detecting the action and behavior of the person.

The large deformation sensor consists of: an elastic band; and a piezoelectric film outputting a signal according to the strain in the longitudinal direction of the elastic band. The large deformation sensor consists of: an elastic band having a Young's modulus of 0.5 to 10MPa; and a pair of piezoelectric films arranged separately in the longitudinal direction of the elastic band and outputting a signal according to the strain in the longitudinal direction of the elastic band. The seated person behavior detection sensor/monitor and the game/exercising apparatus are constituted using them.