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.