PURPOSE: To achieve a reduction in preservation space and a higher position accuracy of a coil for signals with the simplification of setting and exchanging work of the coil for signals, by inserting a cassette coil into an insertion hole inside a mat to arrange the coil for signals of the cassette coil securely within the mat.

CONSTITUTION: An operator removes a stopper 9 of a mat 5 to take out a cassette coil 4 in an insertion hole 6 and inserts another cassette coil 4 to be exchanged into the insertion hole 6 to be arranged securely. Then, a stopper 9 is set at an insertion port 8. This eliminates the need for exchanging the mat 5, which enables the accomplishing of exchanging work of a coil for signals simply in a short time and the setting work is very simple. The number of mats 5 to be preserved is reduced to one and the cassette coil 4 is so thin and small as to allow reduction in preservation space with less capacity thereof, even in the preservation thereof in plurality. In addition, the cassette coils are arranged securely in a mat member 3 with the same shape as the coil so that the centet-thereof 2 is at center positions O of all the mats and moreover, the positions of the cassette coil 4 are adjustable within the insertion hole 6, thereby achieving a higher position accuracy of the coil 2 for signals.