To execute wide-band current detection.

This current detecting device is equipped with a first terminal 101 for receiving a power source current, a second terminal 102 for supplying an external circuit with an output current, a third terminal 103 for outputting a pilot current having prescribed relation with the output current, and a fourth terminal 104 having a reference potential. The device is equipped with a first element 116 connected between the first and the second terminals, cascade connection of first and second baluns 120, 122 coupled with the first element, an earthed capacitive element 124 installed on a connection part of the first and the second baluns, and an earthed capacitive element 128 installed on an output part of the second balun. The performance of an impedance measuring device or a power measuring device can be improved by using the current detecting device.