To provide a sensor that is capable of improving signal strength and detection sensitivity of SAW (Surface Acoustic Waves) and ensuring high operation reliability over a long period of time, and that has a simple structure.

A sensor comprises: a package 13 including a piezoelectric substrate 11 that is formed in the shape of a flat plate and a sealing substrate 12 that is jointed to one surface of the piezoelectric substrate and defines a cavity hermetically sealed between the piezoelectric substrate and the sealing substrate; an IDT (Inter Digital Transducer) 16 that is formed on one surface of the piezoelectric substrate and is arranged inside the cavity; external connection electrodes that are formed on at least one of the piezoelectric substrate and the sealing substrate and are conducted to the IDT through extraction electrodes; and a plate substrate 61 that is jointed to the other surface of the piezoelectric substrate. The piezoelectric substrate has a displacement part that is displaced according to a dynamic quantity affected from the outside of the package 13, and the plate substrate is formed with an opening part E that exposes the displacement part.