Feasibility of Pb_1-x Sn_x Te diode lasers to a high resolution spectroscopy was examined. The laser diode was cooled by liquid helium. Laser emission frequencies were tuned by varying the diode current. A threshold current for cw oscillation by liquid helium cooling was one third or less of that by liquid nitrogen cooling. The lasers oscillated in a few longitudinal modes simultaneously, so a 25cm grating monochromator was used as a mode selection filter. The laser emission from 580cm^-1 to 1450cm^-1 was obtained by changing x=0.14-0.22. Some absorption spectra of NH₃, SF₆ and CF₄, and a gain spectrum of a cw CO₂ laser was observed. Spectral resolution of the experimental equipment was 8×10^-4cm^-1, and it was confirmed that the resolution is mainly limited by the diode driving current.