An image display device comprising: an image signal reception unit configured to receive an image signal of a pattern image in which N partial images, each having a different first gradation level value, are regularly arranged; wherein N is an integer of 2 or more; a conversion unit configured to convert a part of the first gradation level values included in the image signal into a predetermined second gradation level value and to include said predetermined second gradation value in a converted image signal, and is further configured to include in the converted image signal the remaining part of the first gradation level values included in the image signal, wherein said part of first gradation level values that is to be converted is in accordance with a predetermined conversion rule; and a display unit configured to display the converted image signal, wherein the first gradation level value of an n-th partial image among the N partial images is an n-th gradation level value of sequential N-stage gradation levels, for each n ranging from 1 to N; wherein the predetermined second gradation level value is one of the N-stage gradation levels, and the predetermined conversion rule is either: a first conversion rule in which said part of first gradation level values that is to be converted is formed by the even first gradation level values among the first gradation level values included in the image signal and said remaining part is formed by the odd first gradation level values; or: a second conversion rule in which said part of first gradation level values that is to be converted is formed by the odd first gradation level values among the first gradation level values included in the image signal, and said remaining part is formed by the even first gradation level values. an image signal reception unit configured to receive an image signal of a pattern image in which N partial images, each having a different first gradation level value, are regularly arranged; wherein N is an integer of 2 or more; a conversion unit configured to convert a part of the first gradation level values included in the image signal into a predetermined second gradation level value and to include said predetermined second gradation value in a converted image signal, and is further configured to include in the converted image signal the remaining part of the first gradation level values included in the image signal, wherein said part of first gradation level values that is to be converted is in accordance with a predetermined conversion rule; and a display unit configured to display the converted image signal, wherein the first gradation level value of an n-th partial image among the N partial images is an n-th gradation level value of sequential N-stage gradation levels, for each n ranging from 1 to N; wherein the predetermined second gradation level value is one of the N-stage gradation levels, and the predetermined conversion rule is either: a first conversion rule in which said part of first gradation level values that is to be converted is formed by the even first gradation level values among the first gradation level values included in the image signal and said remaining part is formed by the odd first gradation level values; or: a second conversion rule in which said part of first gradation level values that is to be converted is formed by the odd first gradation level values among the first gradation level values included in the image signal, and said remaining part is formed by the even first gradation level values.