A nondestructive inspection apparatus for emitting, to a concrete structure including reinforcing steel bars inside, a neutron beam including at least fast neutrons so that the fast neutrons are repeatedly scattered in the concrete structure to become thermal neutrons, detecting and identifying specific gamma rays generated by reaction between the thermal neutrons and chlorine CI in the concrete structure, and determining a depth at which chlorine CI exists, based on a result of the detecting, the nondestructive inspection apparatus comprising: a neutron source that emits the neutron beam to a surface of the concrete structure; and a gamma ray detection device that detects the specific gamma rays generated due to the neutron beam incident on the concrete structure, wherein the gamma ray detection device includes a gamma ray detector for detecting the specific gamma ray, and a gamma ray shielding portion, a gamma ray passage hole is formed in the gamma ray shielding portion, the gamma ray passage hole includes an opening through which gamma rays are allowed to enter, the gamma ray detector is arranged in the gamma ray passage hole so as to be at a position shifted to a deep side from the opening, the opening and the gamma ray detector are positioned on a reference straight line, the reference straight line is a center line of the gamma ray passage hole, a straight line path of the neutron beam emitted from the neutron source and an extension line of the reference straight line intersect with each other inside the concrete structure, the gamma ray shielding portion includes two shielding blocks (23, 24) separated by a gap from each other in a first direction perpendicular to the reference straight line, and the gap is the gamma ray passage hole, the gap extends along the reference straight line, a size of the gap in the first direction is equal to or smaller than one half, one third, or one fifth of a size of the gap in a second direction perpendicular to both the reference straight line and the first direction, and the gamma ray shielding portion causes the gamma ray detector to selectively detect the specific gamma rays that have been generated in the concrete structure and have entered along the reference straight line from the opening into the gap. a neutron source that emits the neutron beam to a surface of the concrete structure; and a gamma ray detection device that detects the specific gamma rays generated due to the neutron beam incident on the concrete structure, wherein the gamma ray detection device includes a gamma ray detector for detecting the specific gamma ray, and a gamma ray shielding portion, a gamma ray passage hole is formed in the gamma ray shielding portion, the gamma ray passage hole includes an opening through which gamma rays are allowed to enter, the gamma ray detector is arranged in the gamma ray passage hole so as to be at a position shifted to a deep side from the opening, the opening and the gamma ray detector are positioned on a reference straight line, the reference straight line is a center line of the gamma ray passage hole, a straight line path of the neutron beam emitted from the neutron source and an extension line of the reference straight line intersect with each other inside the concrete structure, the gamma ray shielding portion includes two shielding blocks (23, 24) separated by a gap from each other in a first direction perpendicular to the reference straight line, and the gap is the gamma ray passage hole, the gap extends along the reference straight line, a size of the gap in the first direction is equal to or smaller than one half, one third, or one fifth of a size of the gap in a second direction perpendicular to both the reference straight line and the first direction, and the gamma ray shielding portion causes the gamma ray detector to selectively detect the specific gamma rays that have been generated in the concrete structure and have entered along the reference straight line from the opening into the gap.