PURPOSE: To facilitate the discrimination of an unknown electric wave from a known electric wave, by a method wherein a frequency discrimination setting key is provided and a discrimination display mark is displayed on a display apparatus corresponding to the frequency inputted by said setting key to enhance a function as an observation apparatus.

CONSTITUTION: An input signal is subjected to frequency sweeping in a frequency converter part 1 to output an intermediate frequency signal which, in turn, is detected by a detector 3 and digitized by an A/D converter 4 to be accommodated in memory 6. That is, a series of wave forms of spectrum corresponding to the input signal are accommodated in the predetermined address of the memory 6. On the other hand, frequency discriminating data displaying a previously inputted discrimination display mark by a frequency discrimination setting key 8 is accommodated in the address of the memory 6. A microprocessor 5 carries out data treatment for inserting the discrimination display mark 14 in indicated frequency or over an indicated frequency zone on the basis of the information of the frequency discrimination data. By this method, the monitor of an unlawful electric wave or the discrimination of an unknown electric wave and a known electric wave can be directly monitored by eyeview.