Provide is a method for detecting a target substance, which method can visualize the expression of the target substance at any time point while reducing influences on the functions of the target substance, can use fluorescent dyes having various excitation/emission wavelengths, and can achieve easy staining process. Also provided are a tag, a DNA, a vector, a probe and a detection kit suitable for use in the above-described detection method. Specifically, the method for detecting a target substance, comprises the steps of bringing into contact with each other (a) a tag comprising a polypeptide forming an .alpha.-helix structure, the tag bound to the target substance, and (b) a probe comprising a compound bound to a fluorescent dye; and measuring the fluorescence emitted by the fluorescent dye. The binding of the tag .alpha.-helix structure to the probe compound induces a spectral change in the fluorescence emitted by the fluorescent dye.