Group III-V compound semiconductor single crystal substrate and production method therefor

This group III-V compound semiconductor single crystal substrate has a round principal surface. The principal surface has an orientation flat or a notch and has: a first region that is inward of a first imaginary line which passes through a point 5 mm inward from the outer circumference; and a first reference line that is an imaginary line segment which extends from the center of the principal surface to the first imaginary line in a direction from the center of the principal surface to the orientation flat or the notch. The standard deviation and the average value of the number of particles calculated by measuring the number of particles for which particle size at a total of nine measurement points is not less than 0.079 [mu]m satisfy the following relation: standard deviation/average value ≤ 0.9, where the nine measurement points are one point at the center of the principal surface, four center points of four imaginary line segments that have respective angles of 0 degrees, 90 degrees, 180 degrees, and 270