In order to simply realize sound image localization, a method using IIR filters which approximate head-related transfer functions (HRTFs) was proposed previously. However the nonlinear phase characteristics of the IIR filters cause decrease of the localization accuracy. In this paper, we investigate the influence of the phase characteristics on the sound localizatiion. First, we carried out sound image localization experiment using the HRTFs approximated by IIR filters. Next, the experimental errors of directional localization were compared with the theoretical values caluculated from the phase characteristics. As the results, the errors are almost similar to the theoretical values.