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In this letter, we propose a novel approach to voice activity detection (VAD) based on the modified maximum a posteriori (MAP) criterion conditioned on the voice activity decision made in the previous frame. To exploit the inter-frame correlation of voice activity, the probability of the voice presence conditioned on both the observed spectrum and the voice activity decision in the previous frame is employed instead of the conventional strategy that depends only on the current observation. The proposed conditional MAP criterion incorporating temporal correlations leads to two separate thresholds for the likelihood ratio test (LRT) depending on the previous VAD result. Experimental results show that the VAD based on the proposed conditional MAP criterion outperforms the VAD based on the conventional MAP criterion under various noise environments.