PURPOSE: To attain ease of circuit adjustment by providing independently a part making a signal selection switch of the next stage of a detection circuit conductive and a part deciding the timing respectively.

CONSTITUTION: A serial signal comprising plural frequencies is applied to a signal presence discriminating circuit DS1 via tuning fork filters FL50∼FL80 and a signal switch IC1. The switch IC1 selects at first the filter FL50. After a delay circuit X2 delayes an output of the discrimination circuit DS1 for a prescribed time, the X2 counts up to switch the switch IC1 to the filter FL60 of the next stage. When all serial signals are discriminated after the operation above is repeated, a detection output A is given from a counter. On the oher hand, a charge/discharge circuit X1 resets a counter IC2 when a signal of the next section is not inputted within a prescribed time. Since the switching timing of the switch IC1 and the holding time after switching are adjusted independently, the circuit is adjusted easily.