PURPOSE: To measure the absorption by earth's crust of a gravitational wave beam having a sufficient penetration force in the tomography thereof accurately with a sufficient practicability and economy, by using gravitational wave genera tor and detector.

CONSTITUTION: The directivity of a gravitational wave beam from a gravita tional wave generator S set half-stationarily on the surface of the earth's crust is shifted sequentially by a specified angle along the section of the crust to perform a sector scanning. A gravitational wave detector D is shifted sequential ly by a specified angle along the periphery of the crust as viewed from the generator S to detect the gravitational wave beam. Then, a local scanning is done by shifting the direction of spotting the gravitational wave beam emitted from the generator S by a fine angle to the direction orthogonal to the axis thereof as it moves at each specified angle to measure the directivity with and without crust based on the beam power at the center of the gravitational wave beam. Finally, the absorption of the gravitational wave beam by the crust is calculated by a zero method-subtraction and expansion of the measured values by statistical processing.