Abstract: During the accident of the Fukushima Daiichi Nuclear Power Plant in Japan, a large amount of radioactive substance was released into the atmosphere. In order to more precisely evaluate the released quantity, the Lagrangian puff mode was used to deduce the released quantity of ¹³⁷Cs with the plot of ground deposition concentration of ¹³⁷Cs announced by USA and the continuous meteorological data monitored near the Fukushima Daiichi Nuclear Power Plant. The radiological equivalence to ¹³¹I released to the atmosphere is estimated at 1.07×10¹⁸ Bq approximately, which is close to the value published by Japanese government.