The gamma-ray monitor installed at Processing Facility 1 in the JCO Tokai-works recorded the gamma dose rate change over time in proportion to power (nuclear fission rate) in the precipitation vessel from the beginning of the criticality accident to the end of the critical condition. The shape of the gamma dose rate record from 25 min after occurrence to the point of termination ("plateau" part) was normalized using the fission number 2.2 × 1018, and the absolute value of the power was evaluated.