A substance mass measuring system is applied to a microgravitational rotating apparatus vibration control system to easily measure a mass of an object placed in an experimental box. Recess portions (10a, 10b) are provided in a casing (10). Both ends of a rotary shaft (30) are supported by bearings (11, 12) in the recess portions (10a, 10b). A lower end of the rotary shaft (30) is connected to a motor(13). Four arms (24 to 27), arranged horizontally in the direction of X and Y axes, are at their one ends fixed to the rotary shaft (30) and at the other ends fitted with experimental boxes (20 to 23). Experimental objects, such as plants or animals, are placed in the boxes (20 to 23) and rotated for experiments in the space. Vibration of the rotary shaft (30) and the boxes (20 to 23) is absorbed by the bearings (11, 12). Side plates (1a to 1d), acceleration sensors (2a to 2d) and distance sensors (3a to 3d) are arranged in the boxes (20 to 23). By collision of the object with the side plate, mass of the object is computed based on signals from the sensors (2, 3).