An MR/FIR light detector is disclosed herein that has extraordinarily high degree of sensitivity and a high speed of response. The detector includes an MR/FIR light introducing section (1) for guiding an incident MR/FIR light (2), a semiconductor substrate (14) formed with a single-electron transistor (14) for controlling electric current passing through a semiconductor quantum dot (12) formed therein, and a BOTAI antenna (6, 6a, 6b, 6c) for concentrating the MW/FIR light (2) into a small special zone of sub-micron size occupied by the semiconductor quantum dot (12) in the single-electron transistor (14). The quantum dot (12) forming a two-dimensional electron system absorbs the electromagnetic wave concentrated efficiently, and retains an excitation state created therein for 10 nanoseconds or more, thus permitting electrons of as many as one millions in number or more to be transferred with respect to a single photon absorbed.