PURPOSE: To heighten a maximum wave height value, to improve SN ratio, to slim a recording and reproducing signal and to decrease pattern peak shift by twice differentiating the recording and reproducing signal with a differentiation circuit, further controlling gain with a gain control circuit, converting the gain to an echo signal and synthesizing the echo signal with the recording and reproducing signal in an adder circuit.

CONSTITUTION: A first differentiation circuit 12 differentiates the recording and reproducing signal from a magnetic head and a second differentiation circuit 13 differentiates the output signal of the first differentiation circuit 12. Accordingly, the recording and reproducing signal goes to be a signal, which is twice differentiated, and is outputted from the second differentiation circuit 13. A gain control circuit 14 executes gain control to the output signal of the second differentiation circuit 13 and converts the output signal of the second differentiation circuit 13 to the echo signal. Further, this echo signal and the recording and reproducing signal are synthesized by an adder circuit 15. Thus, the recording and reproducing signal to be outputted from the adder circuit 15 is slimed to a shape, in which waveform energy is concentrated, and an SN ratio is improved. Then, the pattern peak shift, which is caused by waveform interference, can be decreased.