A waveform processing circuit includes a switching circuit having first and second inputs and one output. The first input receives head portions and data portions contained in a reproduced signal from an optical head. The second input receives a level signal representing the level of the head portion or the level of the signal carrying the head portion. The switching circuit is switches between a first state in which the first input and the output are connected, and a second state in which the second input and the output are connected. A control signal producer produces a first control pulse indicating the presence of the data portion. The first control pulse is applied to the switching circuit for actuating the switching circuit to establish the first state when the first control pulse is present, and to establish the second state when the first control pulse is not present, so that the switching circuit continuously produces the signal having a level within the range of the head portion. Thus, no abrupt change caused by transient phenomena will be observed in a high pass filter connected to the switching circuit.