Abstract: A phase-shift photo mask blank comprises a half tone phase-shift film, wherein the half tone phase-shift film consists of at least two layers and in the case of two layers, the refractive index of the upper layer of the film is smaller than that of the lower layer thereof; in the case of three layers, the refractive index of the intermediate layer is smaller than those observed for the upper and lower layers or the refractive index of the upper layer is smaller than that of an intermediate layer; in the case of at least 4 layers, the refractive index of the upper most layer is smaller than that of the layer immediately below the upper most layer. The photo mask blank permits the production of a phase-shift photo mask having a high transmittance at an exposure wavelength and a low reflectance as well as a low transmittance at a defect-inspection wavelength. The photo mask in turn permits the fabrication of a semiconductor device having a fine pattern.