To inhibit rotation of a sensor case by excessive rotational force applied thereto, a suction air negative pressure detecting apparatus is structured such that a regular polygonal shaped positioning convex portion is provided on a case containing a pressure sensor, a positioning concave hole having apex angle concave portions at a multiple number of apex angle convex portions of a regular polygon forming the positioning convex portion is provided in a flat-shaped pressure plate arranged on the case, the positioning concave hole is engaged with the positioning convex portion, and the pressure plate is fixed by screw to a throttle body, the case is fixed toward the throttle body by the pressure plate, and a rotation of the case is inhibited by engaging the positioning convex portion with the positioning concave hole.