To measure horizontal permeability in the site ground.

The perimeter of a side wall section 11 having a through-hole 12 of a cylindrical body 10 driven into the ground G of a site is excavated to form a ditch 70. A shaft 60 is excavated in the vertical direction at the center position of the cylindrical body 10. After the excavation of the shaft 60, a cover member 20 is provided to the cylindrical body 10, and water is supplied into the cylindrical body 10 after it communicates with a buret 30a to be separately prepared. At the same time, water is supplied into the ditch 70, and the water level is kept in a certain state. Water is infiltrated into the peripheral surface ground G1 surrounding the shaft 6 from the inside of the shaft 60 in the horizontal direction, and coefficient of water permeability in the horizontal direction is obtained by a specific formula from a permeable amount of water after the peripheral surface ground G1 is saturated.