An environmental energy harvesting device comprising: an energy converting element configured to convert environmental energy into electric energy; an environmental sensor (32, 33) that is disposed in an identical environment as the energy converting element; and a power supply circuit configured to receive electricity converted into by the energy converting element and to output the electricity to an outside, wherein the power supply circuit is configured to change an operation condition in accordance with an output of the environmental sensor (32, 33), wherein the energy converting element is an element configured to generate alternating-current electricity, the power supply circuit includes a rectifier circuit configured to rectify alternating-current electricity converted into by the energy converting element, and the rectifier circuit includes a switching element (M1 to M4) configured to change connection condition for the circuit in accordance with the output of the environmental sensor (32, 33), wherein the environmental sensor (32, 33) is configured to output an alternating-current signal with a frequency equal to a frequency of the alternating-current electricity converted into by the energy converting element, wherein the power supply circuit includes a voltage limiting circuit (40, 40a) configured to limit the alternating-current signal output by the environmental sensor to a voltage within a predetermined range, characterized in that: the power supply circuit includes a current-voltage conversion circuit configured to output a voltage signal in accordance with an amount of current output by the environmental sensor, and wherein the power supply circuit is configured to change a condition of amplification of a voltage to be output to an output part (Vout) of a voltage converting circuit with respect to a voltage received from the rectifier that is input between a first input part and a second input part of the voltage converting circuit based on an output voltage from the current-voltage conversion circuit, so that the efficiency of the environmental energy harvesting device can be further increased. an energy converting element configured to convert environmental energy into electric energy; an environmental sensor (32, 33) that is disposed in an identical environment as the energy converting element; and a power supply circuit configured to receive electricity converted into by the energy converting element and to output the electricity to an outside, wherein the power supply circuit is configured to change an operation condition in accordance with an output of the environmental sensor (32, 33), wherein the energy converting element is an element configured to generate alternating-current electricity, the power supply circuit includes a rectifier circuit configured to rectify alternating-current electricity converted into by the energy converting element, and the rectifier circuit includes a switching element (M1 to M4) configured to change connection condition for the circuit in accordance with the output of the environmental sensor (32, 33), wherein the environmental sensor (32, 33) is configured to output an alternating-current signal with a frequency equal to a frequency of the alternating-current electricity converted into by the energy converting element, wherein the power supply circuit includes a voltage limiting circuit (40, 40a) configured to limit the alternating-current signal output by the environmental sensor to a voltage within a predetermined range, characterized in that: the power supply circuit includes a current-voltage conversion circuit configured to output a voltage signal in accordance with an amount of current output by the environmental sensor, and wherein the power supply circuit is configured to change a condition of amplification of a voltage to be output to an output part (Vout) of a voltage converting circuit with respect to a voltage received from the rectifier that is input between a first input part and a second input part of the voltage converting circuit based on an output voltage from the current-voltage conversion circuit, so that the efficiency of the environmental energy harvesting device can be further increased. the power supply circuit includes a current-voltage conversion circuit configured to output a voltage signal in accordance with an amount of current output by the environmental sensor, and wherein the power supply circuit is configured to change a condition of amplification of a voltage to be output to an output part (Vout) of a voltage converting circuit with respect to a voltage received from the rectifier that is input between a first input part and a second input part of the voltage converting circuit based on an output voltage from the current-voltage conversion circuit, so that the efficiency of the environmental energy harvesting device can be further increased.