A target substance-detecting apparatus for detecting a target substance in an analyte comprises a detecting device having a substrate and metal structures capable of causing a plasmon resonance arranged thereon, an illumination optical system for illuminating an incident light containing a polarization component polarized in a first direction onto the detecting device to cross the arranged metal structures, a light-receiving device for receiving light containing the polarization component polarized in the first direction and transmitted through or reflected by the detecting device, and an arithmetic unit for calculating on signals from the light-receiving device to analyze a characteristic of the analyte; the metal structures arranged at first intervals not more than 1/10 of a plasmon resonance wavelength in lines in the first direction and at second intervals not less than ¼ of the plasmon resonance wavelength and not more than the wavelength in lines parallel in a second direction.