A substance detection sensor comprises an insulation layer (2) having flexibility, two electrodes (3A, 3B) so disposed on the insulation layer as to be opposite to each other at a space therebetween and connected to an electric resistance detector, and a conductive layer (4) which is so formed on the insulation layer as to span the two electrodes and to be electrically connected to the two electrodes. The swelling ratio of the conductive layer (4) is changed according to the type and/or the quantity of a specific substance.