To high precisely and readily specify a connection position of a branch pipe connected to a drainage main pipe from inside a spiral pipe which is a lining pipe.

In a state that an impedance head 12 fitted on an oscillator 11 is brought into pressing contact with an inner peripheral face of a spiral pipe 20, if the oscillator 11 is oscillated under a sine wave AC voltage, the oscillation of the oscillator 11 is transmitted to oscillate the spiral pipe 20. The impedance head 12 detects a magnitude of an outer force applied on the spiral pipe 20 by the oscillator 11, and outputs to a calculation control part 17 via a charge amplifier 15, and also detects an acceleration of the oscillation generated in the spiral pipe 20 and outputs to the calculation control part 17 via a charge amplifier 16 for converting into an oscillation speed. The calculation control part 17 calculates a mechanical admittance which is an oscillation speed per unit outer force based on the outer force applied on the spiral pipe 20 and the oscillation speed generated in the spiral pipe 20.