[en] For TARN (Test Accumulation Ring for Numatron), two different pick-up monitors for a stochastic cooling system are considered: a helix type and a ferrite type. For the helix type probe, the calculation of the coupling impedance, which is defined by the ratio of the voltage that is induced in the probe and the beam current, has already been performed. The coupling impedance of a ferrite type probe may be derived in the similar way, however, the formulas are more complicated. The particle beam which is surrounded by a ferrite core produces a magnetic flux in this core. The time variation of this flux induces a voltage in the pick-up coil. In the paper, the derivation of the equations for the electric field for a small ferrite (metal wall matching), a magnetic field and the pick-up voltage, beam current and the coupling impedance, field matching at the ferrite core and discussion on the matching condition are described. The results of the calculation are shown in several figures. The conclusion of these calculation is as follows: in order to achieve a high coupling impedance by the use of a ferrite pick-up probe, the ferrite core should be as close to the beam as possible; it is desirable to choose a material with a smooth decrease of μ versus frequency. Under these conditions, an improved coupling impedance as compared to that of the helix monitor is achievable. (Wakatsuki, Y.)