The present invention relates to a well type neutron counter containing a He-3 detector which includes at least one annular gas layer in a polyethylene moderator, which includes a body formed of a neutron moderator and having a sample cavity for inserting a sample of nuclear material therein; and an annular He-3 detector tube including at least one annular gas layer into which at least He-4 or He-3 gas or their mixture is injected and a plurality of anode rods stood in the annular gas layer with an equal spaces, wherein the He-3 detector tube is formed in an inside of the body so as to surround the sample cavity. The neutron counter has a largely reduced size, simplified structure and resultant decreased failure rate as compared to a conventional counter with a large volume.