[en] Pulse height distributions were measured using a CdZnTe detector inside a lead collimator to investigate main source producing high dose rates above the shield plugs of Unit 3 at Fukushima Daiichi Nuclear Power Station. It was confirmed that low energy photons are dominant. Concentrations of ¹³⁷Cs under 60 cm concrete of the shield plug were estimated to be between 8.1*10⁺⁹ and 5.7*10⁺¹⁰ Bq/cm² from the measured peak count rate of 0.662 MeV photons. If ¹³⁷Cs was distributed on the surfaces of the gaps with radius 6 m and with the averaged concentration at 5 points equal to 2.6*10⁺¹⁰ Bq/cm², the total amount of ¹³⁷Cs is estimated to be 30 PBq. The results of our measurements show that a larger amount of ¹³⁷Cs than emitted into the environment at the time of accident, exists between the gaps of the shield plugs