The lifetime of carriers in TiSea is determined from the resistivity data by using the theories of excitonic insulators. The comparison with infrared measurements shows that the size of the discontinuity in the temperature-gradient of the resistivity at T_c is in accord with the theoretical prediction for excitonic insulators. The estimated transition-temperature for a perfect crystal of TiSe_2 nearly agrees with the onset of the strong short-range-order effect which is observed in the resistivity well above T_c. These results support the excitonic-insulator mechanism for the phase transition in TiSe_2.