The present invention provides a method for detecting a target substance by detecting the presence or concentration of a target substance in a sample solution by bringing the sample solution into contact with a detecting element including a detecting part and a non-detecting part and detecting the presence or number of a magnetic label (magnetic marker) present in the vicinity of the surface of the detecting part and provides a target-substance detection kit. The surface potential ψ1 of the magnetic label in the sample solution, the surface potential ψ2 of the detecting parts and the surface potential ψ3 of the non-detecting part satisfy any one of the following relationships i) to iv): i) ψ1ψ3>0 and ψ2=0, ii) ψ1ψ2 0, and |ψ2|>|ψ3|, and iv) ψ1ψ2