There is provided in an embodiment according the present invention, a signal processing device comprising detecting means for detecting a predetermined frequency component of the input signal; modulating means for modulating the input signal and supplying the modulated signal to a recording device; and switching means for supplying, in a first state, the output of the modulating means to the detecting means, and for supplying, in a second state, a reproduced signal to the detecting means from a reproducing device. With the aforementioned construction, the detection circuit that detects a predetermined frequency component included in the modulated digital signal can be used during recording as well as during signal reproduction, which greatly contributes to reducing the size and cost of the recording/reproducing device.