A semiconductor dynamic quantity sensor has a substrate including a semiconductor substrate, an insulation layer on a main surface of the semiconductor substrate, and a semiconductor layer on the insulation layer. The main surface has a projection that is trapezoidal or triangular in cross section. The semiconductor layer is divided by a through hole into a movable portion. A tip of the projection is located directly below the movable portion and spaced from the movable portion by a predetermined distance in a thickness direction of the substrate. A width of the tip of the projection is less than a width of the movable portion in a planar direction of the substrate. The distance between the tip of the projection and the movable portion is equal to a thickness of the insulation layer.