In the present study, the intelligibility of the synthesized speech sounds was evaluated by using the psycholinguistic and fMRI techniques. In order to see the difference in recognizing words between the natural and synthesized speech sounds, word regularity and word frequency were varied. The results of Experiment1 and Experiment2 showed that the intelligibility difference of the synthesized speech comes from word regularity. In the case of the synthesized speech, the regular words were recognized slower than the irregular words, and there was smaller activation of the auditory areas in brain for the regular words than for the irregular words.