Abstract: A device for controlling transmission of electromagnetic waves according to the present disclosure includes: a conductor line which is positioned on a signal layer and through which electromagnetic waves received via an input terminal travel; a ground layer electrically separated from the signal layer through a dielectric layer and electrically grounded; a shunt via including a first end and a second end and connected to the conductor line through the first end; and a photoconductive semiconductor connected between the second end of the shunt via and the ground layer and having a dielectric state or a conducting state, based on an input of an optical signal, wherein the conductor line is electrically connected to the ground layer via the shunt via and the photoconductive semiconductor in the conducting state, thereby causing reflection of electromagnetic waves from the shunt via.