To provide a biosensing method for detecting a target DNA at low cost simply in high sensitivity.

The biosensing method for detecting a target DNA comprises: modifying a PCR primer of the target DNA with a mediator for transferring electrons caused by a reaction between an enzyme and a substrate in an electrolyte to which an electric current is applied; subjecting the target DNA to PCR amplification using the mediator-modified primer; adding a PCR product and a substrate to an electrolyte equipped with an enzyme immobilized electrode, applying an electric current to the electrolyte; measuring an electric signal from the enzyme immobilized electrode and, when an electric current is detected; and determining that the target DNA exists in an analysis sample.