PURPOSE: To precisely observe the deterioration of an eye pattern by judging a quadrant arranged on a vector plane by means of a reception signal, rotating the reception signal in accordance with a judged result and outputting the rotated signal to the prescribed quadrant.

CONSTITUTION: An equalizer 1 removes noise from the received signal and outputs the signals of a real component showing the coordinate value and an imaginary component. A quadrant rotation part 3 judges in which quadrant on a coordinate vector plane an input signal exists from the real component and the imaginary component of the inputted signal, and executes a rotation processing so that a signal point is arranged in a first quadrant in accordance with the result. An origin shift part 4 shifts the origin of the coordinate of the signal point which the quadrant rotation part 3 rotates. An enlargement part 5 enlarges the plane of the signal point coordinate which the origin shift part 4 origin-shifts. A D/A-converter 6 converts a digital signal into an analog signal, and the output is connected to an oscilloscope for displaying the eye pattern through an output terminal.