An optical frequency converter includes a waveguide including a core made of a nonlinear optical medium having a refractive index n1,light in a wavelength region of light, and cladding disposed so as to cover the core and made of a material whose refractive index n2,light in the wavelength region of light is lower than the refractive index n1,light, and a coupling section made of a material whose refractive index n3,THz in a wavelength region of a terahertz wave is higher than the refractive index n1,light and disposed in contact with the cladding. The coupling section is configured to couple the waveguide with a space in the wavelength region of the terahertz wave. The coupling section covers the cladding.