The present invention provides a sheet mass measuring device capable of improving productivity and precisely measuring the mass of sheets. In the present invention, the mass measuring device (10) includes two driving rollers (2U, 2L) vertically separate from each other and intersecting the direction of conveyance of a mail matter. Servo motors (6U, 6L) for driving the driving rollers (2U, 2L) are connected to torque detection units (8U, 8L). A control unit controls motor control units (7U, 7L) to cause them to impart a rate difference between the driving rollers to rotate the mail matter during its conveyance, and to cause the torque detection units (8U, 8L) to detect the torque values of the servo motors (6U, 6L). The control unit compares the thus detected torque values with the torque values of the servo motors detected when the servo motors are idling without rotating the mail matter, thereby measuring the mass of the mail matter using a calculation unit.