Fuel cell (FC) is a clean power source and does not emit CO 2 at its electric power generation. In recent years, low voltage and large current type FC stack is becoming popular in some applications. Thus, a DC/DC converter with the high voltage-boost ratio for such FC stack is required and several types of converters have been researched. As one of such DC/DC converters with the high voltage-boost ratio, there is a cascaded three-stage boost converter. In this converter, it is necessary to consider the combinations of duty ratios which can obtain higher efficiency with respected to change in FC output voltage. Therefore, the efficiency characteristics of the cascaded boost converter are clarified by experiments. In an actual system, there are some losses such as switching losses and the copper losses in the inductors and the capacitors. In addition, 1st stage duty ratio $d_{1}$ was set to 0.5 in order to obtain minimum input current ripple. Therefore, the duty ratios of 2nd and 3rd stage converters must be set larger value than the theoretical value. In the experiments, power conversion efficiencies were measured for each combination of the duty ratios. As a result, in order to obtain high power conversion efficiency, it is preferable that the duty ratio $d_{2}$ in the 2nd stage converter is larger while the duty ratio $d_{3}$ in the 3rd stage converter is smaller.