PURPOSE: To provide an apparatus for three-axial compression test in situ whereby a strength constant of the ground at the place can be simply and correctly adjusted at the spot.

CONSTITUTION: An apparatus for three-axial compression test in situ is constituted of a pressure bag 2 comprising a rubber sleeve 2b, a device 3 for adding a restraint pressure to the bag 2, and a loading device 4 disposed above a sample 1 on the same axial line for loading an axial load. The rubber sleeve 2b is attached to the inner periphery of a metallic cylindrical body 2a which holds and restrains the cylindrical sample 1 cut out from the original ground.