The present invention prevents pressure from being exerted locally and minimizes generation of dust from a gripping part when a mask blank substrate is gripped by a transfer robot. This mask blank substrate 10 is provided with: first and second main surfaces 12a, 12b; four lateral surfaces 16a-16d; first to fourth chamfered faces 18a-18d that are formed between the first main surface 12a and the four lateral surfaces 16a-16d; and fifth to eighth chamfered faces 18a'-18d' that are formed between the second main surface 12b and the four lateral surfaces 16a-16d. When the seventh chamfered face 18c' is defined as a reference plane Pc' in a cross-section A that is substantially perpendicular to the first and second main surfaces 12a, 12b and the two lateral surfaces 16a, 16c, the parallelism of an outline La of the first chamfered face 18a situated in a diagonal direction from the seventh chamfered face 18c' is at most 0.02 mm. When the first chamfered face 18a is defined as a reference plane Pa, the parallelism of an outline Lc' of the seventh chamfered face 18c' is at most 0.02 mm.
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