PURPOSE: To speedily and easily determine a phase correction quantity by displaying a phase rotation quantity at each peak of an absolute value spectrum and the phase correction quantity based on a phase correction coefficient.

CONSTITUTION: An absolute value of each data is calculated by a main computer 13 and a fast Fourier transform circuit 15 finds an absolute value spectrum according to the absolute value. The absolute value spectrum is displayed on a display device 16 and the phase rotation quantity at the peak is also displayed on the display device 16. The phase correction quantity straight line is displayed on the display device 16 immediately, so an operator varies parameters over a look at it to find the best phase correction quantity.