Analysis of the operation modes of an RF-Field-Driven DC-SQUID (RFDS) is presented. We numerically calculate the current-voltage characteristics (IVC) of the RFDS, where the RF signal is coupled to the SQUID loop magnetically. Under no DC offset flux, the IVC exhibit the enhancement of the even-order steps. We first evaluate the dependence of the maximum 2nd step height of the RFDS upon frequency. Contrary to the results for a single junction, the RFDS maintains its step height at a certain value in the low frequency region. The maintained values of the maximum step height are dependent on βL. The smaller βL is, the larger the maximum step height becomes. Next, we evaluate the dependence of the current positions of the 2nd step upon the amplitude of the RF signal. Under the low frequency condition, the current positions agree with the interference patterns of the SQUID, which means that the operation of the RFDS is based on the quantum transitions in the SQUID loop. Under the high frequency condition, on the other hand, the current positions agree with the results for the single junction, which means that the quantum transitions does not follow the RF signal and that the RFDS behaves like a single junction.