Electrochemical impedance spectroscopy (EIS) is an excellent method to determine simultaneously many parameters concerning electron-transport processes in dye-sensitized solar cells under the same conditions as those for the cell in operation. A number of studies analyzed the electron-transport processes based on EIS measurements under dark conditions, although the photocurrent density-voltage (I-V) measurements were carried out under one-sun illumination conditions. The result of EIS measurement obtained under dark conditions, however, is quite different from that obtained under illumination. The EIS measurement should thus be carried out under the same illumination conditions as the I-V measurement is performed to obtain properly analyzed results in regard to the electron-transport processes. The main factor contributing to such difference between dark and illumination conditions was elucidated to be the difference in the electron density in the steady-state cell performance between dark and illumination conditions.