To reduce influences of impulses, by holding sensitive instruments against impulses like a transmitter for positional detection by separating them from the chamber wall containing instruments to the main impulse direction with attenuating cushions against impulses.

A transmitter 14 for positional detection is fitted in the cylindrical chamber 6 in the housing 2 of a boring machine 1. Transmission slits 7 are provided in the chamber 6 to transmit signals from the transmitter 14. The transmitter 14 is provided in the cylindrical housing 2 with a larger diameter than the chamber 6. And a cushion 15 is provided at the front end of the transmitter 14 turning toward the manipulating head 9 and a shock-absorbing cushion 16 is provided at the end turning toward the rear end. Silicone rubber with 15-30 in Shore hardness is used for the cushions 15, 16 and provided with a ring-form groove 17 and the axial tenon 18 to guarantee the deforming function.