Infrared optical part and method of making the same

An infrared optical part is comprised of a synthetic diamond having a nitrogen content of not more than 5 ppm and a boron content of not more than 3 ppm, wherein the parallelism between its light incident and reflecting surfaces is not more than one minute. The infrared optical part is used as a window member in infrared spectral analysis. It is also used in the form of a pair of anvils for holding a sample therebetween in connection with measurement of transmitted light that has passed through the sample after the sample is compressed. the part is also used in the form of an infrared ATR prism. The method of making the infrared optical part comprises the steps of abrading an infrared optical part (11) having the above mentioned nitrogen and boron contents by a grinder, measuring the parallelism between the light incident and reflecting surfaces (17,18) of the part by light from a laser (14), and smoothing the grinding apparatus, whereby the parallelism is set to a level of not more than one minute.