PURPOSE: To enable the detection of an explosive without suffering the effect of nitrogen existing in air, by a method wherein the air in an ambience surrounding a substance to be inspected which is located at an inspecting position is replaced by a substance not containing the nitrogen and neutrons are applied to the substance to be inspected.

CONSTITUTION: An annular inspection stage 7a supported rotatably by a plurality of rollers 7d is located in an annular space which is formed between a support 7 provided in the center of a cylindrical housing 5 serving also as a radiation shield, and the housing 5. A partitioning body 7b is provided on the central side of the inspecting stage 7a, and a plurality of vertical partitioning plates 7c project radially therefrom. A plurality of holes 8a are made in the support 7 and a carbonic acid gas supply pipe 8 is made to communicate with these holes 8a. A neutron source 1 and a neutron moderator 4 are provided in a region under the inspecting stage 7a, while a gamma ray detector 2 is provided in a region above the partitioning body 7b. A substance 6 to be inspected is set in a sectorial region between the partitioning plates 7c. When the substance 6 to be inspected arrives at an inspecting position, inspection is executed by a detector 2 and an output signal thereof is analyzed by a data processing device 3.