A physical quantity sensor includes a first substrate, an electrode provided on the first substrate, a diaphragm made of semiconductor material, a second substrate fixed to the first substrate, a dielectric film provided on the diaphragm, and a wall provided between the dielectric film and the electrode. The second substrate supports the diaphragm such that the diaphragm has an opposing surface facing the electrode across a space. The dielectric film is provided on the opposing surface of the diaphragm. The dielectric film has a surface facing the electrode across the space. The wall includes a first protrusion and a second protrusion. The first protrusion protrudes toward the electrode from the surface of the dielectric film. The second protrusion protrudes toward the electrode from the first protrusion, and contacts the electrode. The second protrusion is made of material which is different from material of the dielectric film.