Provided are a power supply device, a power receiving device, a charging system, and an obstacle detection method that obtain a sufficient obstacle detection sensitivity even when an obstacle is small. A modulation unit (202) performs amplitude modulation or phase modulation on a test data sequence output from a test data sequence storage unit (201). A power control unit (203) generates, according to an instruction from a determination unit (204), a power control signal for increasing the power level of the test data sequence every time when the test data sequence is transmitted. The determination unit (204) determines whether there is an obstacle between the power receiving device and the power supply device based on whether the test data sequence output from the test data sequence storage unit (201) coincides with the test data sequence output from a power-transmitting-side receiving circuit (124).