A modification method of a carbon nanotube sheet comprising: mounting at least one carbon nanotube sheet on an assembly; and exposing the carbon nanotube sheet on the assembly to steam or particles of a substance that is liquid at room temperature, wherein the carbon nanotube sheet has a structure in which a plurality of carbon nanotubes are aligned preferentially in one direction in a plane of the sheet, the assembly comprises a mounting section on which the carbon nanotube sheet is mounted, the mounting section comprises a non-contacting section that is not brought into contact with the carbon nanotube sheet and a contacting section that is brought into contact with the carbon nanotube sheet, L 1 is defined by a maximum distance across the non-contacting section between intersection points of a straight line extending in parallel with an alignment direction of the carbon nanotubes in a plan view of the mounting section and crossing over the non-contacting section with a border between the non-contacting section and the contacting section, while L 2 is defined by a maximum distance across the non-contacting section between intersection points of a straight line crossing over the non-contacting section and intersecting with the alignment direction of the carbon nanotubes in the plan view of the mounting section with the border between the non-contacting section and the contacting section, when L 1 is larger than L 2, at least L 2 is more than 10 μm and 5 mm or less, when L 1 is smaller than L 2, at least L 1 is more than 10 μm and 5 mm or less, and when L 1 is equal to L 2, each of L 1 and L 2 is more than 10 μm and 5 mm or less. mounting at least one carbon nanotube sheet on an assembly; and exposing the carbon nanotube sheet on the assembly to steam or particles of a substance that is liquid at room temperature, wherein the carbon nanotube sheet has a structure in which a plurality of carbon nanotubes are aligned preferentially in one direction in a plane of the sheet, the assembly comprises a mounting section on which the carbon nanotube sheet is mounted, the mounting section comprises a non-contacting section that is not brought into contact with the carbon nanotube sheet and a contacting section that is brought into contact with the carbon nanotube sheet, L 1 is defined by a maximum distance across the non-contacting section between intersection points of a straight line extending in parallel with an alignment direction of the carbon nanotubes in a plan view of the mounting section and crossing over the non-contacting section with a border between the non-contacting section and the contacting section, while L 2 is defined by a maximum distance across the non-contacting section between intersection points of a straight line crossing over the non-contacting section and intersecting with the alignment direction of the carbon nanotubes in the plan view of the mounting section with the border between the non-contacting section and the contacting section, when L 1 is larger than L 2, at least L 2 is more than 10 μm and 5 mm or less, when L 1 is smaller than L 2, at least L 1 is more than 10 μm and 5 mm or less, and when L 1 is equal to L 2, each of L 1 and L 2 is more than 10 μm and 5 mm or less.