When electromagnetic waves with two frequencies that differ by a factor of two propagate in a metallic waveguide, it is difficult to propagate only the TE10 mode without the TE20 mode for higher frequency region. We propose a TE10-mode-only source using two wave sources whose frequencies differ by a factor of two with a frequency-dependent mode converter. The TE10-mode source is composed of an oscillator section and a mode converter section. Gunn diodes of 10 and 20 GHz are used to generate the electromagnetic waves. The oscillator section also has five dielectric rods centered in a metallic waveguide in order to pass the TE10 mode at 10 GHz and suppress the TE10 mode at 20 GHz without influencing the TE20 mode. In the mode converter section the 10-GHz TE10 mode is passed and the 20-GHz TE20 mode is converted to the TE10 mode, so that the output is solely TE10 mode. We experimentally confirmed the functioning of the converter.