PURPOSE: To improve a processing speed on the whole by providing a means which performs mutual correlation arithmetic operation as to not all data, but some frequency data when operating the mutual correlation between data converted to a frequency range by data compression processing and reference frequency data.

CONSTITUTION: Data are compressed by the 1st data processing part 15 or the 2nd data processing part 16 by choice. The 2nd data processing part 16 performs mutual correlation arithmetic processing as to all frequency components of received data and its processing time is long, but obtained data have an excellent SN ratio and a reproduced image is sharp. The 1st data processing part 15, on the other hand, sections the frequency components of the received data into plural parts and compresses them; and the number of processed data is much smaller than that of the 2nd data processing part 16. Consequently, the number of times of arithmetic operation for Fourier transform and inverse Fourier transform is decreased greatly.