This paper presents the basic theoretical background of frequency domain decomposition (FDD) technique considering the estimation error caused by close modes. On the basis of the background, a FDD technique for proportional damping systems is proposed. The technique identifies the modal parameters from only the real part of the power spectrum density function matrix of response. In order to validate the proposed technique, numerical simulations with the conventional FDD technique and the proposed technique are carried out. The results show that the accuracy of the proposed technique is higher than that of the conventional one.