A detection system for detecting a state change of an inspection target, comprising: a sensing device including: a vibration unit for applying vibration to the inspection target, the vibration unit configured to be attachable to the inspection target, a driving circuit for supplying an electrical signal to the vibration unit for driving the vibration unit, and a sensor for detecting vibration of the inspection target caused by the vibration applied from the vibration unit; and a detection processing device for receiving vibration information related to the vibration of the inspection target detected by the sensor from the sensing device and detecting the state change of the inspection target based on the vibration information, wherein the vibration unit of the sensing device includes a case in form of a cylindrical member comprising a cover, a base, which is configured to be attachable to the inspection target, and a cylindrical portion located between the cover and the base, and the vibration unit further includes a coil fixedly attached on the base and in which the electrical signal supplied from the driving circuit flows, a leaf spring provided so as to be capable of vibrating with respect to the case, and a magnet assembly comprising a magnet and being attached to the leaf spring so as to be apart from the coil, wherein the case contains the coil, the leaf spring and the magnet assembly therein, the leaf spring has a ring shape with an outer peripheral portion held between the base and the cylindrical portion of the case, and a central portion containing a central opening portion, wherein the magnet assembly is attached to the central portion, and wherein the central portion can vibrate with respect to the base so that the magnet assembly can vibrate with respect to the coil, the case comprises three extending portions formed on an outer peripheral surface of the base so as to extend in a radial direction of the base and through-holes respectively formed in tip side portions of the three extending portions, wherein the base can be fixed to the inspection target by means of screws passed through the through-holes of the base and respectively screwed with screw holes formed in the inspection target, the coil has a cylindrical shape and is located inside the central opening portion of the leaf spring, and the magnet assembly includes a magnet holding portion having a cylindrical shape opening toward a lower side, the magnet fixed on a central lower surface of the magnet holding portion and a yoke attached to a lower surface of the magnet, wherein the magnet and the yoke are arranged in a central hollow portion of the coil so as to be apart from the coil. a sensing device including: a vibration unit for applying vibration to the inspection target, the vibration unit configured to be attachable to the inspection target, a driving circuit for supplying an electrical signal to the vibration unit for driving the vibration unit, and a sensor for detecting vibration of the inspection target caused by the vibration applied from the vibration unit; and a vibration unit for applying vibration to the inspection target, the vibration unit configured to be attachable to the inspection target, a driving circuit for supplying an electrical signal to the vibration unit for driving the vibration unit, and a sensor for detecting vibration of the inspection target caused by the vibration applied from the vibration unit; and a detection processing device for receiving vibration information related to the vibration of the inspection target detected by the sensor from the sensing device and detecting the state change of the inspection target based on the vibration information, wherein the vibration unit of the sensing device includes a case in form of a cylindrical member comprising a cover, a base, which is configured to be attachable to the inspection target, and a cylindrical portion located between the cover and the base, and the vibration unit further includes a coil fixedly attached on the base and in which the electrical signal supplied from the driving circuit flows, a leaf spring provided so as to be capable of vibrating with respect to the case, and a magnet assembly comprising a magnet and being attached to the leaf spring so as to be apart from the coil, wherein the case contains the coil, the leaf spring and the magnet assembly therein, the leaf spring has a ring shape with an outer peripheral portion held between the base and the cylindrical portion of the case, and a central portion containing a central opening portion, wherein the magnet assembly is attached to the central portion, and wherein the central portion can vibrate with respect to the base so that the magnet assembly can vibrate with respect to the coil, the case comprises three extending portions formed on an outer peripheral surface of the base so as to extend in a radial direction of the base and through-holes respectively formed in tip side portions of the three extending portions, wherein the base can be fixed to the inspection target by means of screws passed through the through-holes of the base and respectively screwed with screw holes formed in the inspection target, the coil has a cylindrical shape and is located inside the central opening portion of the leaf spring, and the magnet assembly includes a magnet holding portion having a cylindrical shape opening toward a lower side, the magnet fixed on a central lower surface of the magnet holding portion and a yoke attached to a lower surface of the magnet, wherein the magnet and the yoke are arranged in a central hollow portion of the coil so as to be apart from the coil.