An electromagnetic wave detector 100 comprises: a substrate 5 having a front surface and a back surface; an insulating layer 4 formed of a rare earth oxide, which is provided on the front surface of the substrate 5; a pair of electrodes 2 provided on the insulating layer 4 so as to be arranged to face each other across a gap; and a two-dimensional material layer 1 provided on the insulating layer 4 so as to be electrically connected to the pair of electrodes 2. The rare earth oxide contains a base material made of an oxide of a first rare earth element, and a second rare earth element different from the first rare earth element, which is activated in the base material.