Milk of four of the mums tested has shown positive for radioactive iodine.
The highest reading, 36.3 becquerels of radioactive iodine per kg was found in the milk of a mum of an eight-month-old baby in Kashiwa.
While there is no current legal safety level for radioactive substances in breast milk, the milk readings that were taken were below the safety limit of 100 becquerels per kg of tap water consumed by children under one year of age.
The findings have started a debate about the impact of the nuclear disaster on mums and babies.
'We cannot yet determine safety, but infants drink breast milk,' said Kikuko Murakami from the citizen's group who conducted the study on the breastfeeding mums. 'We want the Government to conduct an extensive investigation swiftly.'