PROBLEM TO BE SOLVED: To ensure a high sensitivity measurement even in a millimetric wave or sub-mellimetric wave region by constituting a measuring equipment of a Fabry-Perot resonator and one or two dielectric waveguide being inserted into the Fabry-Perot resonator in order to introduce or lead out wave energy.
SOLUTION: The resonator with dielectric waveguide has such a structure as one or two dielectric waveguide for introducing or leading out wave energy is inserted into one or two hole made in the reflector of a Fabry-Perot resonator. The resonance wavelength L is set generally 2-50 times, most preferably 5-6 times, as long as the wavelength. Fabry-Perot resonator having two concave reflection surfaces or one concave surface and the other planar surface is employed, for example. The Fabry-Perot resonator is designed to satisfy an expression; W14=(λ/π)2b12L/(b1-L), W24=(λ/π)2L/(b1-L), where λ is wavelength at resonant frequency, L is length of resonator b1 is radius of curvature of spherical mirror, W1 is spot size on spherical mirror, and W2 is spot size on planar mirror.