We developed an insulator-based dielectrophoretic (iDEP) creek-gap device that enables the isomotive movement of cells and that is suitable for determining their DEP properties. In the iDEP creek-gap device, a pair of planar insulators forming a single fan-shaped channel allows the induction of the isomotive iDEP force on cells. Hence, the cells' behavior is characterized by straight motion at constant velocity in the longitudinal direction of the channel. Operation of the device was demonstrated using human breast epithelial cells (MCF10A) by applying an AC voltage of V-pp = 34V peak-to-peak and frequencies of 200kHz and 50MHz to the device. Subsequently, the magnitude of DEP forces and the real part of the ClausiusMossotti (CM) factor, Re(), were deduced from the measured cell velocity. The values of Re() were 0.14 +/- 0.01 for the frequency of 200kHz and -0.12 +/- 0.01 for 50MHz. These results demonstrated that the DEP properties of the cells could be extracted over a wide field frequency range. Therefore, the proposed iDEP creek-gap device was found to be applicable to cell analysis.