In disposal of high-level radioactive waste, carbon steel overpack will be corroded after closure of the repository, creating a reducing, low-pH environment around the repository. A plutonium diffusion experiment was performed over 15 years with Kunigel V1, which is a typical Japanese bentonite that contains about 50% montmorillonite, in contact with an iron coupon. A tracer solution (10 µL) containing 1 kBq of 238Pu was applied at the interface between the iron coupon and compacted bentonite that was saturated with deionized water. After the diffusion period, the plutonium distribution in the bentonite specimen was measured with an alpha scintillation counter, and the iron and sodium distributions were obtained by inductively coupled plasma-mass spectrometry. Plutonium penetrated into the bentonite to a depth of 2 mm, and more than 90% of plutonium remained in corrosion product at the interface. The bentonite around the interface was dark green like green rust or magnetite according to visual observation. Iron was detected throughout the bentonite and there was a particularly high iron concentration from the interface to a depth of 2 mm, whereas the sodium concentration decreased slightly from the interface to 2 mm. We proposed that ferrous ions diffused into bentonite as the iron coupon corroded and precipitated such as magnetite. The magnetite precipitation would decrease the bentonite pH, resulting in the dissolution and migration of plutonium. Small cracks were also observed 2 to 6 mm from the interface and could have been formed by the migration of hydrogen generated by corrosion.