Since an immersion grating provides n (n: refractive index of its material) times higher spectral resolution compared to a conventional reflective grating of the same size, an immersion grating is a powerful optical device for the infrared high-resolution spectrometer. Recently a high-resolution spectrometer in the infrared wavelength range is increasing the importance increasingly for observations of relating with H2O, NHx, NOx and organic molecules. Higher spectral resolution allows us to detect weak lines without spectral line confusion. On the other hands, there is no practical immersion grating for high-resolution spectrometer except Si immersion grating by anisotropic etching. It was very difficult for a fragile IR crystal to manufacture a diffraction grating precisely by machining. Our original free-forming machine has accuracy of a few nano-meter in positioning and stability. We succeeded in fabricating immersion gratings with three kinds of materials. Three materials are CdZnTe, germanium and InP, each refractive index is about 2.7, 4.0 and 3.2 respectively. By combining these devices, a spectrometer with immersion grating is realizable in the wavelength range of 1.5-20um. Thereby, the realization of these immersion gratings has led to a dramatic improvement in the operability and performance of next generation high-performance spectrometer. In this paper, we report performance of our immersion gratings and other possibility.