PURPOSE: To allow the self-advancing device to advance accurately at a fixed distance while various probes are held in a boring pit, etc., and to correspond even to not only a vertical pit but also an inclined pit and a horizontal pit.

CONSTITUTION: The self-advancing device is composed of first and second blocks 10, 12 and an expansion drive section 14 connecting the blocks. A plurality of stages of a plurality of leaf spring-shaped leg piece sections 16 are all protruded to the outer circumferential surfaces of both blocks uniformly in the same oblique outward direction. The expansion drive section is expansion-contraction- moved in the axial direction. The front ends of the leg piece sections reach a boring pit wall. Since the front ends of the leg piece sections push the pit wall, the device is held in the pit. Since each leg piece section is directed in the same oblique upward direction, resistance at the front ends of the leg piece sections is increased in the upward direction, thus alternately moving both blocks only in the downward direction.