A concentration detector includes: a neutron source emitting neutrons to a target; a gamma ray detector detecting and determining an amount of specific gamma rays that are among gamma rays generated in the target by interactions with the neutrons; and a concentration calculator calculating a concentration of the target at selected depths in the inspection target, based on the detected amount. A relational expression expressing a relation between a plurality of concentrations of the target in a plurality of virtual layers and a detected amount of the specific gamma rays is predetermined for each type of the specific gamma rays or each detection condition. The concentration calculator applies the detected amount for each gamma ray type or each detection condition, to the relational expression for the type or the detection condition, and calculates a concentration of the target component in the layer at each depth or the specific depth.