PURPOSE: To measure the gravity without considering a resistance of the atmosphere by placing orthogonally a rigid rod in which mass bodies having the same mass are fixed to both its ends in its center and constituting it so as to be free turnable and detecting the torque by a turning angle of the rod.

CONSTITUTION: When gravity works on two pieces of mass bodies 1a, 1b having the same mass which are fixed to both ends of a rigid rod 2, the rod 2 rotates in the direction around a rotation axis 3. In this state, a turning angle of this rod 2 is detected by a detector 4, and its detection value is given to a rebalance circuit 6. The circuit 6 controls a torquer 5 so that the turning angle becomes zero. A moving amount in the circuit 6 in this case comes to be proportional to the torque applied to the mass bodies 1a, 1b, therefore, from this controlled variable, gravity can be derived.