Applicability to a composite material with high purity and high strength, and a material requiring high conductivity or high thermal conductivity is enhanced. The present invention relates to a multi-walled carbon nanotube having two or more tubes of a graphene sheet where carbon atoms are arranged in a hexagonal honeycomb form, coaxially, wherein a diameter of an outermost wall based on observation of an image by a transmission electron microscope is 3 nm or more and 15 nm or less, and a length based on observation of an image of a scanning electron microscope is 1.0 mm or more, an aggregate of multi-walled carbon nanotubes and a method for preparing the multi-walled carbon nanotube.