To collect a soil sample with an arbitrary length till a necessary depth without recovering a core tube of a sampler on the ground.

A top end bit 3, the core tube 2, and a coupling head 5 are screwed to the sampler 1. The core tube 2 is separated from the coupling head 5 when the sampler 1 reaches a predetermined excavation depth. A new core tube 2 is incorporated by mounting an extension coupling 11 between the core tube 2 and the coupling head 5. The excavation is continued by reconnecting the core tube 2 with the coupling head 5, and the soil sample 22 is taken into the core tube 2. The soil samples 22 with an arbitrary length are sampled without being restricted by a length of the single core tube 2 by repeating a series of works. When the sampler reaches a predetermined depth, each soil sample 22 is collected per the single core tube by pulling up the core tubes 2 and disconnecting them.