A test signal generating part (3) generates test signals of four points where two pairs of points each consist of two points that are point symmetric about the origin point on an I/Q rectangular coordinate system. An envelope determining part (8) determines the amplitudes of output signals of a quadrature modulator at the generation of the four-point test signals, and outputs signals that are proportional to the respective squares of those amplitudes. A comparing part (9) calculates the average value of the output signals from the envelope determining part (8) for each pair of test signals when that pair of test signals are generated. A control part (10) adjusts the amplitudes and/or phases of the test signals in such a mariner that the average values are equal to each other at the generation of two pairs of test signals, and calculates, based on the adjustment result, an I/Q mismatch amount.