PURPOSE: To accurately measure gap hydraulic pressure without causing a hydraulic pressure sensor to be affected by a fine soil particle by simultaneously measuring each gap hydraulic pressure of a plurality of water-keeping layers by a drilling hole.

CONSTITUTION: In a protection pipe 1, a plurality of connection protection pipe bodies 2 and pipes 12 for measuring distortion are connected by an upper blocking members 3 and a lower blocking members 5. A plurality of hydraulic pressure measurement chambers 4, 4... are demarcated in axial direction while being separated by the upper blocking member 3 and the lower blocking member 5 inside the protection pipe 1. A hydraulic pressure sensor 6 is provided inside each hydraulic pressure measurement chamber 4. Each hydraulic pressure measurement chamber 4 communicates with a drilling hole A by water-guiding holes 8, 8,... formed at the protection pipe 1. A hair-shaped filter part is provided around the water-guiding holes 8 on the outer-periphery surface of the protection pipe 1. Also, annular water-shielding seal parts for shielding a plurality of water-guiding holes 8 communicating with the hydraulic pressure measurement chambers 4 are provided while being separated in axial direction on the outer-periphery surface of the protection pipe 1.