A mass sensor includes a diaphragm, a sensing plate having a piezoelectric element arranged on at least a part of at least one surface joining respective sides, a connecting plate sandwiched by the diaphragm and the sensing plate, wherein the diaphragm, the sensing plate, the piezoelectric element, and the connecting plate form a resonating portion. The connecting plate is bridged across the side surfaces of a concave portion formed in a sensor substrate, and the sensing plate is joined to at least the bottom portion of the concave portion. Change in the mass of the diaphragm is measured by measuring change in the resonant frequencies of the resonating portion accompanying the change in the mass of the diaphragm. The mass sensor enables the easy and accurate measurement of a minute mass of a nanogram order including microorganisms such as bacteria and viruses, or chemical substances, or the thickness of vapor-deposited films.