An energy-harvesting device includes a power generating element configured to generate electric power, a storage capacitor configured to store the electric power generated by the power generating element, a DC/DC converter configured to convert, into a predetermined voltage, a voltage obtained from the electric power generated by the power generating element and the electric power stored in the storage capacitor, a delay section for delaying a voltage corresponding to a voltage across the storage capacitor, and a controller configured to output an activating signal that activates the DC/DC converter when the delayed voltage is higher than a reference voltage. The energy-harvesting device thus can produce a necessary voltage and power.