A new 320 GHz solid-state source interferometer is installed in the Heliotron J helical device to explore the physics of high-density plasmas (ne > 2–3 × 10¹⁹ m⁻³, typically) realized with advanced fueling techniques. This interferometry system is of the Michelson type and is based on the heterodyne principle, with two independent solid-state sources that can deliver an output power of up to 50 mW. A high time resolution measurement of <1 µs can be derived by tuning the frequency of one source in the frequency range of 312–324 GHz on the new system, which can realize the fluctuation measurement. We successfully measured the line-averaged electron density in high-density plasma experiments. The measured density agreed well with a microwave interferometer measurement using a different viewing chord, demonstrating that the new system can be used for routine diagnostics of electron density in Heliotron J.