PURPOSE: To supply always a measuring reference signal at a prescribed level without error by a measuring desired frequency component only by providing a function transmitting its level measuring signal transmission to a signal frame having a highway transmitting and receiving a signal through the transmission/ reception of a time slot and a time division switch.

CONSTITUTION: The signal frame 10 transmitting and receiving a conventional MFC signal or the like is connected to the time division switch 3, a signal of a reference level is superimposed on a part of a time slot on the highway between the switch 3 and the frame 10 from a reference signal generator 11 in the signal frame and transmitted to the switch 3. The switch 3 extracts the time slot of the measuring desired frequency by the time division control of a controller 5 and transmits the time slot to a measuring line by the control of time slot replacement. In connecting a selecting level meter 11 to an output side of the measuring line, the level measurement of the desired frequency is attained for many times by having only to match the frequency of the level meter to any of frequencies 6, 7, 8 and 9. Further, the measuring signal having no frequency error with the signal on the preceding measuring line and of the level without level error is transmitted to other measuring line by the control of the controller 5.