A method of manufacturing an elongated metal plate used for manufacturing a deposition mask having a plurality of through-holes formed in the metal plate, the method comprising: a rolling step of rolling a base metal to obtain the metal plate having an average value of plate thicknesses in a longitudinal direction within a ±3% range around a predetermined value; a cutting step of cutting off one end and the other end of the metal plate in a width direction over a predetermined range; a first inspection step of measuring a plate thickness of the elongated metal plate along the longitudinal direction of the metal plate at plural points; a second inspection step of measuring the plate thickness of the elongated metal plate along the width direction of the metal plate at plural points; and a selection step of selecting the elongated metal plate which satisfies all the below conditions and: when an average value of the plate thicknesses of the metal plate in the longitudinal direction is represented as A, and a value obtained by multiplying a standard deviation of the plate thicknesses of the metal plate in the longitudinal direction by 3 is represented as B, (B/A) × 100 (%) is 5% or less; and when a value obtained by multiplying a standard deviation of the plate thicknesses of the metal plate in the width direction by 3 is represented as C, and a value of a plate thickness of the metal plate at a central portion in the width direction, which is obtained when plate thicknesses of the metal plate are measured along the width direction in order to calculate the standard deviation of the plate thicknesses of the metal plate in the width direction, is represented as X, (C/X) × 100 (%) is 3% or less. a rolling step of rolling a base metal to obtain the metal plate having an average value of plate thicknesses in a longitudinal direction within a ±3% range around a predetermined value; a cutting step of cutting off one end and the other end of the metal plate in a width direction over a predetermined range; a first inspection step of measuring a plate thickness of the elongated metal plate along the longitudinal direction of the metal plate at plural points; a second inspection step of measuring the plate thickness of the elongated metal plate along the width direction of the metal plate at plural points; and a selection step of selecting the elongated metal plate which satisfies all the below conditions and: when an average value of the plate thicknesses of the metal plate in the longitudinal direction is represented as A, and a value obtained by multiplying a standard deviation of the plate thicknesses of the metal plate in the longitudinal direction by 3 is represented as B, (B/A) × 100 (%) is 5% or less; and when a value obtained by multiplying a standard deviation of the plate thicknesses of the metal plate in the width direction by 3 is represented as C, and a value of a plate thickness of the metal plate at a central portion in the width direction, which is obtained when plate thicknesses of the metal plate are measured along the width direction in order to calculate the standard deviation of the plate thicknesses of the metal plate in the width direction, is represented as X, (C/X) × 100 (%) is 3% or less. when an average value of the plate thicknesses of the metal plate in the longitudinal direction is represented as A, and a value obtained by multiplying a standard deviation of the plate thicknesses of the metal plate in the longitudinal direction by 3 is represented as B, (B/A) × 100 (%) is 5% or less; and when a value obtained by multiplying a standard deviation of the plate thicknesses of the metal plate in the width direction by 3 is represented as C, and a value of a plate thickness of the metal plate at a central portion in the width direction, which is obtained when plate thicknesses of the metal plate are measured along the width direction in order to calculate the standard deviation of the plate thicknesses of the metal plate in the width direction, is represented as X, (C/X) × 100 (%) is 3% or less.