The input load characteristics of a DC motor vary with input powers. The conversion efficiency of a rectifying circuit is strongly affected by the connected load. Therefore, to efficiently drive a DC motor using microwave power transfer (MPT), it needs to improve the efficiency-load characteristics of the rectifying circuit. We design a compact power-receiving device which mounts antennas, rectifiers and a buck-boost converter on a multilayer substrate. The buck-boost converter exhibits constant input resistance characteristics independent of the load resistance. Experiments are carried out on driving a low-power DC motor using MPT. High transfer efficiency is achieved between the receiving antenna array and the input of the DC motor. In particular, the measured overall efficiency of the power-receiving device is over 62 % as the power density is changing from 0.25 to 1 mW/cm(2).