To reliably gather a core by projecting a lower end part of a core housing bit in an inner part downward from the lower end of an excavating bit of an outer part, and transmitting rotation of the outer part to the core housing bit in a condition where a water passing passage is secured in the outer part and the inner part.

An outer part 4 of a boring device part 2 connects an outer tube 4B to the lower end of a lot 4A having a prescribed dimension, and installs an excavating bit 4C on its lower end tip. An inner part also connects a latch 5C to the lower end of a suspending metal fitting 5B arranged in a latch retracting case 5A, and connects an inner tube 5F to the lower end of a latch body 5D connected to its lower end, and installs a core housing bit 5G on its lower end. Here, a lower end part of the core housing bit 5G projects downward from the lower end of the excavating bit 4C, and the outer part 4 and the inner part transmit rotation of the outer part 4 to the core housing bit 5G in a condition where a water passing passage is secured.