A mass sensor in which a connection plate and a diaphragm are joined together at their respective sides; a sensing plate is joined to the connection plate at their respective sides in the direction perpendicular to the direction where the diaphragm is joined to the connection plate; a piezoelectric element consisting of a piezoelectric film and an electrode is installed on at least either one of plate surfaces of the sensing plate; and a resonating portion consisting of the diaphragm, the sensing plate, the connection plate, and the piezoelectric element is joined to a sensor substrate. Change in the mass of the diaphragm is measured by measuring change in the resonant frequency of the resonating portion accompanying the change in the mass of the diaphragm.