A halftone phase shift photomask blank comprising a transparent substrate and a halftone phase shift film consisting of silicon, nitrogen and optional oxygen, and providing a phase shift of 150°-200° relative to light of wavelength up to 200 nm. The phase shift film includes at least one layer meeting the formula: 2×O/Si+3×N/Si≧3.5 wherein Si is a silicon content (at %), N is a nitrogen content (at %), and O is an oxygen content (at %). The phase shift film exhibits satisfactory in-plane uniformity of transmittance.