Light having a wavelength to be absorbed by a target object is irradiated from a light irradiating unit toward a subject while varying its intensity in the course of time. At least two detecting elements arrayed along a scanning direction detect a component having a large time delay out of the light propagated through the subject, for example, as departing from the light irradiating unit. By comparing measured data on the basis of the light detected by the at least two detecting elements with measured data in the case where the target object is not present in a light irradiation region, the depth of the target object at a location of the light irradiating unit is acquired.