An optical detection type chemical sensor includes a light source, a detection element and a photodetector. The detection element is constituted of a laminate in which a multilayer film including a chemical detection layer, an optical interference layer, and a half mirror layer is formed on a transparent substrate. At least one of the layers includes a magnetic material. Light from the light source is applied to the detection element under the condition that the light enters inside of the detection element from the rear surface of the transparent substrate on which the laminate is not formed and multiple reflection occurring in the laminate intensifies the magneto-optical effect. A subject is detected by using the photodetector to detect a magneto-optical signal indicating a change in reflected light from the laminate resulting from a change in an optical property resulting from a reaction in the chemical detection layer.