Abstract: A sensor includes a detector including an element substrate, a membrane including an outer surface, an inner surface on an opposite side of the outer surface, and a diaphragm, a side wall provided on the element substrate and supporting a portion of the inner surface of the membrane outside the diaphragm, and a fixed electrode provided on the element substrate and surrounded by the side wall, facing the inner surface of the membrane with a space therebetween and having an electrostatic capacitance between the fixed electrode and the diaphragm. A first recess portion is provided in the outer surface of the membrane between the center of the diaphragm and the side wall in the thickness direction of the membrane.