Sensor apparatus

A sensor apparatus, comprising:

• an element substrate comprising a first top surface, a second top surface and a third top surface;
• a detecting section disposed on the first top surface of the element substrate, the detecting section comprising a reaction section having an immobilization film disposed on and directly contacting the first top surface, wherein the immobilization film is configured to detect an object to be detected, a first IDT electrode disposed on the second top surface of the element substrate and configured to generate an acoustic wave which propagates toward the reaction section, and a second IDT electrode disposed on and contacting the third top surface of the element substrate and configured to receive the acoustic wave which has passed through the reaction section;
• and
• a protective film which covers the first IDT electrode and the second IDT electrode,
• wherein the first top surface of the element substrate is at a lower level than the second and third top surfaces of the element substrate.