PURPOSE: To constitute an image indicating the positions in both horizontal and vertical directions of an input image by detecting the spacial periodicity of a certain specific period adjustable in the horizontal direction included in the input image by applying a correlation calculation and to display the output image by the same horizontal scanning as the horizontal scanning speed for the input image.

CONSTITUTION: An acoustooptic correlator 6 using an ultrasonic optical modulator and a Fourier transform optical system executes the square-law cross-correlation calculation of two input signals in real time. The video signal of an input image 1 obtd. by an image pickup device 2 is fed through a synchronizing pulse sepn. circuit 3 which removes horizontal and vertical synchronizing pulses to a square-law compression circuit 4 which performs square-law compression of signal amplitude, the output from which is applied as one input signal to the correlator 6. A reference signal generator 5 generates a reference signal of the periodic and symmetrical waveform having a specific frequency and inputs the same as the other input signal to the correlator 6. The output signal from the correlator has the amplitude to express the quantity of the component of the time frequency included in the video signal. Said signal is inputted together with the horizontal and vertical sweep signal outputted from the image pickup device to an output image display device 7 which constitutes and displays the image indicating the position where the component of the frequency is detected.