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; and a cutting step of cutting off one end and the other end of the metal plate in a width direction over a predetermined range; wherein the metal plate is made of an invar material containing 34 to 38% by weight of nickel and has a plate thickness of 80 μm or less; and wherein the following two conditions and are satisfied as to a variation in plate thickness of the metal plate: 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; and a cutting step of cutting off one end and the other end of the metal plate in a width direction over a predetermined range; wherein the metal plate is made of an invar material containing 34 to 38% by weight of nickel and has a plate thickness of 80 μm or less; and wherein the following two conditions and are satisfied as to a variation in plate thickness of the metal plate: 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.