PURPOSE: To obtain data in a short time, by measuring the beat of a reference signal having the same frequency increasing ratio as that of an interference fringe signal at estimated acceleration due to gravity and the beat of an interference fringe signal obtained during falling of an actual body.

CONSTITUTION: A ray emitted from a stabilized laser source 101 collides against a semitransparent mirror 102, a part thereof goes to a fixed reflector 103, reflects thereon and returns. The other ray passes through the mirror 102, collides against a moving reflector 104 provided at the falling body, reflects and returns. These both rays are added at the mirror 102 into an optical and photoelectric converter 105 and the synthesized rays cause mutal interfere to generate the interference fringe, thus inputting it as a electric signal. By producing the beats of this output signal and a reference signal generator (figure is omitted), recording can be performed in real time by a recorder (figure is omitted).