We have performed neutron tomography using two ancient Japanese swords (designated Morikage and Sukemasa) and one modern Japanese sword (Masamitsu) at RADEN in the J-PARC Materials and Life Science Experimental Facility. For the ancient Japanese sword Morikage, it is found that the martensite iron is distributed in the region of about 3 mm from the cutting-edge and the ferrite iron is distributed in the inner region of the blade. The martensite iron area surrounds the inner ferrite iron area. For the ancient Japanese sword Sukemasa, the martensite iron is distributed only in a very narrow region at the cutting-edge and the homogeneous ferrite iron area is dominantly distributed in the inner region of the blade. In contrast to the ancient Japanese swords, the distribution of the martensite iron is about 8 mm from the cutting-edge and is similar to the wave pattern visible on the blade surface for the modern Japanese sword Masamitsu. Additionally for Masamitsu, a region where the neutron transmittance slightly increases was found at the interface between the martensite area and inner ferrite iron area. Line-like structures due to inclusions produced in the manufacturing process were also found in the blade. These results indicate that the manufacturing processes and raw materials of the Japanese swords are significantly different depending on the era and place of manufacture. Going forward, it is necessary to compare systematically the internal structure of more samples in order to clarify historical changes in Japanese sword making, and the nondestructive approach using neutron tomography is one of the powerful tools to elucidate them.