A mechanical quantity sensor includes a first structure having a fixed portion with an opening, a displaceable portion arranged in the opening and displaceable relative to the fixed portion, and a connection portion connecting the fixed portion and the displaceable portion, a second structure having a weight portion joined to the displaceable portion and a pedestal joined to the fixed portion, and arranged and stacked on the first structure, and a base having a driving electrode and a detection electrode arranged on a face facing the weight portion, connected to the pedestal, and arranged and stacked on the second structure. The second structure has a projection arranged in an area on a face of the weight portion facing the base, the area corresponding to an area where the driving electrode and the detection electrode are not arranged, the projection having a thickness larger than thicknesses of the driving electrode and the detection electrode.