Spatial aliasing is a challenging issue that faced by most multiple speech source localization methods. Small-size arrays are widely used to avoid or mitigate spatial aliasing. But they deteriorate the coherence in low frequencies and degrade the performance of localization. This paper proposes a phase difference regression method for multiple speech source localization on a planar array. The time delay histogram is firstly applied to classify the frequency bins into clusters that correspond to speech sources, and then, the phase difference regression is conducted on each cluster. Since the error of the phase difference is limited in the range of [−π, π], the proposed method avoids the ambiguity in the period number of phase. Although conventional regression method considers the period number, it does not bring significant advantage over the proposed method. The experimental results confirm the superiority of the proposed method on large-size arrays.