An electromagnetic wave generating device is provided which includes an optical waveguide including a plurality of waveguide segments such that the main lobe of a combined electromagnetic wave has a substantially single large directivity.The electromagnetic wave generating device includes the optical waveguide including a plurality of waveguide segments each of which is sandwiched between dielectrics and includes a nonlinear optical crystal. The waveguide segments are arranged such that an angle formed by the directions of propagation of light in the two adjacent waveguide segments substantially corresponds to 2θc. When ng denotes the refractive index of the nonlinear optical crystal for light and ∈eff denotes the effective relative permittivity of an assembly of the dielectrics and the waveguide segments for an electromagnetic wave, θc is defined as θc=cos−1 (ng/θ√∈eff).