A manufacturing method of a Blazed diffraction grating configured to diffract incident light and made of a CdTe or CdZnTe crystal material includes the step of forming a plurality of grating grooves in a processed surface of a work through machining using a processing machine for the Blazed diffraction grating. The forming step forms the grating grooves so that among surfaces of gratings formed by the forming step, a surface that receives the incident light most is set to a (110) plane as a crystal orientation of the crystal material.