PURPOSE: To make the measurement of mass possible even in a gravity-free space by having the first spring whose stretching vibration is generated through a measured article attached to a heavy weight and a weight which is attached to the heavy weight symmetrically to the measured article through the second spring and vibrated while balancing with the measured article through the regulation of its mass.

CONSTITUTION: The first spring 3 is attached at its basic end to a heavy weight 1 vibratably provided in a gravity-free space, and a measured article 5 attached to the tip of the first spring 3 generates the stretching vibration of the first spring 3. A weight 6 is attached to the heavy weight 1 symmetrically to the measured article 5 through the second spring 3, and vibrated while balancing with the measured article 5 by regulating its mass. The mass of the weight 6 is regulated in the gravity-free space, and the measured article 5 and the weight 6 are simultaneously accelerated at the same rate for their vibration. The vibrations of the first and the second spring system can be thereby balanced to make the heavy weight 1 keep a stationary state, and the mass of the measured article 5 can be measured from the then mass of the weight 6.