A fluorescence sensor comprises a sensor section for collecting a fluorescent substance, which acts to represent presence of a substance to be detected in a sample, and an exciting light source, which produces exciting light for exciting the fluorescent substance to produce fluorescence. Besides the exciting light source, at least one different non-exciting light source is located for irradiating different non-exciting light, which varies in wavelength from the exciting light and which is substantially free from capability of exciting the fluorescent substance, to the sensor section.