Simultaneously oscillated multi-line far infrared (FIR) lasers from a single FIR laser cavity have been developed as optical sources of a two-color interferometer for plasma diagnostics. Using two pump CO₂ lasers which oscillated at the different wavelength, we have obtained simultaneously oscillated 119-μm laser pumped by 9P(36) CO₂ laser and 57-μm laser pumped by 9R(8) CO₂ laser from mixed gases of CH₃OH and CH₃OD. Common beam splitters of the two-color FIR laser interferometer have been designed by using precise optical constants (refractive index and absorption coefficient) of silicon.