A powder density measuring device where a pair of purge gas-introducing paths is provided in a direction transverse to a material-measuring path in which pneumatically transported powder is introduced, and a pair of light transmission windows is provided so that the material-measuring path is viewable through the pair of the purge gas-introducing paths. A light projector is provided at one of the light transmission windows and a light receiver is provided at the other window so as to be opposed to each other. Powder density calculating/controlling means is provided, and, when a light beam radiated from the light projector is radiated to inside the material- measuring path, the means calculates powder density based on the amount of light received by the light receiver.