In a quadrature modulation device which receives a pair of carrier signals whose phases are orthogonal to each other and I and Q signals and outputs a modulation signal, a quadrature error between phases of the pair of carrier signals is detected. What is added to the I and Q signals is a pair of such DC voltages that a modulation signal output from the quadrature modulation device when the I and Q signal levels are zero is at a predetermined level. A plurality of such combinations of values of a pair of DC voltages are searched that the signal level of the modulation signal is at the predetermined level when the pair of DC voltages are respectively changed and successively added to the I, Q signals. The quadrature error is calculated from a simultaneous equation having as variables the values in the searched plurality of combinations of values of a pair of DC voltages, the signal level of the modulation signal, and the quadrature error of the carrier signal.