The invention provides a method of sensing a target object due to a change in light intensity detected when the target object passes to cross a light path of circularly polarized light, in a state where circularly polarized light selectively including any one sense of right circularly polarized light and left circularly polarized light is sensed, and a system including: an emission unit which emits circularly polarized light; a target object movement unit; and a detection unit which senses circularly polarized light, in this order, in which the detection unit is at a position where light emitted from the emission unit is incident, a light path of light where the light emitted from the emission unit is incident to the detection unit intersects with the target object movement unit, and the sense of circularly polarized light selectively emitted by the emission unit and the sense of circularly polarized light selectively sensed by the detection unit are the same as each other. It is possible to sense various target objects by using an optical sensor with excellent sensitivity and decreased erroneous sensing in an arbitrary environment, by using the method and the system of the invention.