A method of eliminating unnecessary frequency components using a quadrature detector. The method includes converting an analog input signal into a digital signal f1(t), and the signal f1 (t) is delayed by a sampling time .tau. to form a signal f2 (t). Then, letting the reference frequency be .omega.0, the I- and Q-components in quadrature detection by the following expression: I={f2(t)*sin .omega.0t-f1(t)*sin .omega.0(t-.tau.)}/sin .omega.0.tau. Q={f2(t)*cos .omega.0t-f1(t)*cos .omega.0(t-.tau.)}/sin .omega.0.tau..