To solve the problem of the measurement of long-term ground displacement since the fracture of time dependency of the ground due to long-time stress, namely creep fracture, occurs through a certain determined creep deformation process of the ground and hence the stability of the ground can be evaluated and the fracture time of the ground can be predicted to some degrees by measuring the deformation of the ground for a long term and grasping a speed change in the movement of the ground, but the long-term displacement measurement of the ground that extremely slowly moves due to the ground creep cannot be made by the conventional technique.

Both the ends of a measurement probe are fixed to an immovable ground 1 and a movable ground 2 in a boring hole 4 by a grout 7, and the displacement between the fixing is measured with time by an optical fiber displacement sensor 8 built into the probe. The probe is made of stainless steel to withstand a long-term measurement, and a slide section is provided for expansion and contraction. Additionally, the conventional optical fiber sensor that has a high measurement precision and is rich in durability is used.