To provide a neutron detector equipped with a boron trifluoride gas proportional counter, capable of obtaining a neutron detection sensitivity determined beforehand, while generation of fluorine gas is suppressed.

Gas pressure for enclosing boron trifluoride gas is reduced, and the boron trifluoride gas is enclosed in a boron trifluoride gas proportional counter element 1 at the gas pressure of about 250 mmHg. In order to compensate a neutron detection sensitivity deteriorated by reduction of the gas pressure, the boron trifluoride gas proportional counter elements 1 are connected in parallel, and the two boron trifluoride gas proportional counter elements 1 are stored in an assembly case 5, to thereby compose a boron trifluoride gas proportional counter 6.