The objective of the present invention is to provide a versatile method that makes highly sensitive detection of nucleic acid, and the like, possible even if a solution has a high salt concentration. This objective has been achieved through a method for detecting a target substance in a solution through EIS impedance measurement, wherein the solution includes at least the target substance, an impedance observation substance A, and a substance B for causing the charge mobility of the impedance observation substance to vary, and the substance B for causing the charge mobility to vary has a property of being taken into the target substance.