Passive optical networks (PONs) have been used conventionally.  In PONs, a branching apparatus is inserted in an optical fiber network to branch a single optical fiber to plural subscribers.  An optical line terminal (OLT) at a station in a PON receives burst optical signals transmitted intermittently from optical network units (ONUs) of subscribers via a star coupler. 
When an OLT detects a burst optical signal, a method using a peak detecting circuit and a comparator is commonly performed (see, e.g., Japanese Laid-Open Patent Application No.  2000-115218).  The peak detecting circuit outputs a peak current representing a peak of a current received at the OLT.  The comparator compares output from the peak detecting circuit with an SIGDET threshold.  When the result of comparison by the comparator indicates that the output from the peak detecting circuit is equal to or higher than the SIGDET threshold, a signal is detected by the OLT. 
The conventional technique above, however, poses a problem in that erroneous detection of a signal during a state of no signal reception may occur if no signal is received for a long period.  Specifically, because the peak detecting circuit is a kind of an integrating circuit that remains irreversible unless reset (i.e., circuit with no output decrease), output from the peak detecting circuit rises gradually due to noise even during a state of no signal reception.  The longer the state of no signal reception, the greater the possibility that output from the peak detecting circuit will exceed the SIGDET threshold.  Therefore, if a signal is not received for a long period, output from the peak detecting circuit exceeds the SIGDET threshold, resulting in erroneous signal detection.