PURPOSE: To detect a tracking state exactly and easily, by fetching a part on which a data of a transmission signal is not superposed, and detecting a noise contained in this part.

CONSTITUTION: A PCM signal applied to an input terminal 1 is supplied to a gate circuit 3 through a buffer circuit 2. The gate circuit 3 is selectively switched by a clamp pulse supplied from an inpt terminal 4, and a pedestal part is fetched to its output side only while the clamp pulse is applied. A noise component superposed on this pedestal part is supplied to a high-pass filter 5, and unless tracking of a VTR exists at an optimum position, the noise component of its output side is also increased. This noise component is supplied to a detecting circuit 8 through a high-pass filter 7 from an amplifying circuit 6, and is supplied to a meter 10 through a level regulator 9. In order to prevent a detection error in case when the PCM signal is not supplied from the input terminal 1, an AGC circuit 11 is provided.