Provided is a mass sensor having a four-corner-adjustment mechanism capable of reducing the sensitivity of four-corner adjustment without lowering the rigidity of a Roberval mechanism. A mass sensor (1) includes a Roberval mechanism (2) in which a floating frame (21) and fixed part (22) are coupled by upper and lower sub-rods (23, 24) having thin parts (23b) formed thereon, and a four-corner-adjustment mechanism (4) for parallelization. The four-corner-adjustment mechanism (4) has an adjustment screw (41) and an upper elastic member (42u) and lower elastic member (42d) that are disposed in series in the vertical direction so as to sandwich the sub-rod (23) subjected to parallelization.