A 15‐channel far‐infrared (FIR) interferometer system has been constructed to measure the electron densities in the Advanced Toroidal Facility (ATF) device. The system consists of a pair of cw 214 μm difluoromethane (CH2F2) lasers, optically pumped by separate CO2 lasers. The large number of channels is achieved by the use of reflective beam expansion optics to create a beam of 2 cm×45 cm. After passing through the plasma discharge, the beam is dissected by an array of 15 off‐axis paraboloid reflectors, each of which illuminates a single Schottky‐diode detector. The system can also be operated at a wavelength of 119 μm for high density experiments by changing the gas in FIR laser cavities to methanol. Details of the system are described and the recent results of the measurements are presented.