In a physical quantity sensor, a first substrate has a recess depressed from a second surface to provide a thin film section adjacent to a first surface, and a second substrate has a first surface bonded to the first surface of the first substrate, and has a hollow depressed from the first surface and facing the recess. The recess and the hollow have such sizes that a projected line defined by projecting an end of a bottom surface in the recess to the first surface of the first substrate surrounds an open end of the hollow. When the thin film section is displaced toward the hollow, a maximum tensile stress is generated at a position on a rear surface of the thin film section intersecting an extended line along a normal direction to the first surface of the first substrate and passing through the open end of the hollow.