According to the present invention, an electromagnetic wave detection device includes an optical waveguide, an electromagnetic wave input unit, and a phase difference measurement unit. According to the thus constructed electromagnetic wave detection device, an optical waveguide is a nonlinear crystal, and includes a branching portion for receiving a probe light pulse, and causing the probe light pulse to branch into two beams of branching light, and two branching light transmission portions for receiving the branching light from the branching portion, and transmitting the branching light. An electromagnetic wave input unit inputs an electromagnetic wave having a frequency equal to or more than 0.01 [THz] and equal to or less than 100 [THz] tilted by an angle generating Cherenkov phase matching with respect to a travel direction of the branching light into one of the two branching light transmission portions.