If a duty ratio, a load resistance and an input voltage in a boost DC-DC converter are changed, the dynamic characteristics are varied greatly, that is, the boost DC-DC converter has non-linear characteristics. In many applications of the boost DC-DC converters, the loads cannot be specified in advance, and they will be changed suddenly from no load to full load. In the boost DC-DC converter, a conventional single controller cannot be adapted to change dynamics and it occurs large output voltage variation. In this paper, an approximate 2-degree-of-freedom (A2DOF) digital controller for suppressing the change of step response characteristics and the variation of an output voltage in load sudden change is proposed. Experimental studies using a micro-processor for the controller demonstrate that this type of digital controller is effective to suppress the variations of the output voltages.