A method and apparatus detect a noise disappearance of instruments, etc. in various instrumentations in a chemical plant or the like. Detection values which are outputs from a detector (1) are input into a data store (3) via a data collector (2). Inputs to a processor (4) are a detection value (Sold) at a previous point and time from the data storer (3) and further a current detection value serving as a newest value (Snew) from the data collector (2). In the processor (4), an absolute difference between the newest value (Snew) and the previous value (Sold) is calculated. When this difference is smaller than a preset value, a count number (C) of a counter (5) is incremented, whereas when this difference is larger than the preset value, the count number (C) is cleared. When the count number from the counter (5) exceeds a predetermined threshold value, this is regarded the occurrence of a noise disappearance, and a trigger signal (T) for actuating a notifier (7) is output from a limiter (6).