To provide a technique capable of preventing a core from being washed away together with slime by excavation water and collecting a geological sample in a fine-grained ground such as a gravel ground and a clay ground, or in a soft ground in a fine shape under a natural condition.

When the core of the geological sample is collected from inside the ground by a core barrel 1 provided with a cylindrical core bit 4, and a cylindrical outer tube 2 with the core bit 4 fixed on its tip, and having a core storage part communicated with a cylinder inside of the core bit 4 inside, the core is collected the core storage part through an excavation water supply passage 2a provided to be separated from the core storage part while drilling the ground by rotating the core barrel 1, while supplying the excavation water to a cylinder outside side of the core bit 4, and while injecting the water from a downstream end part of the water supply passage 2a in the direction toward the tip of the core bit 4.