A target substance-detecting apparatus comprises a target substance-detecting element comprising metal structures, a light irradiation section for irradiating the target substance-detecting element with a light, a light-polarizing section which polarizes the irradiating light and separates an output light emitted from the target substance-detecting element into a first polarized light and a second polarized light, first and second light-receiving sections for outputting first and second signals according to intensity of the first and second polarized lights, respectively; and a control section which determines peaks of absorbances of the first and second polarized lights by measuring the absorbances from the first and second signals respectively, and controls the target substance-detecting element so that the peak values of the first and second absorbances can be maximized and minimized respectively by controlling a incidence angle of formed by a vibration direction of the incident light and a main axis of the target substance-detecting element.