PURPOSE: To make the complicated transmission of a signal exact and easy by laying down an insulating liquid pipe, to which insulating liquid is charged, between first and second points, between which the signal is transmitted, converting the transmitted signal to a liquid pressure signal at the first point and converting the a liquid pressure signal to a signal to be requested at the second point.

CONSTITUTION: A joint 1 of a machine, angle/length conversion part 2 and length/liquid pressure conversion part 4, etc., are provided at a first point A and a liquid pressure/length conversion part 10 and length/electricity conversion part 14 are provided at a second point B. Between the conversion parts 4 and 14, an insulating liquid pipe 8 of structure, for which insulating liquid 6 is charged in a main body 9 of the insulating pipe, is laid down. At the point A, the mechanical angle of the joint 1 is converted to the liquid pressure signal through the conversion parts 2 and 4 and the signal is sent through the pipe 8 to the point B. At the point B, the signal is converted to an electric signal by the conversion parts 10 and 14.