To obtain an addition picture added at an optical addition ratio.

When picture signals S1 and S2 obtained from the surface and back face of a stimulable phosphor sheet are inputted to an adding means 1, a table T is read from a table storing means 2 in which a plurality of addition ratios are stored as a table T and displayed on a display means 3. When an operator selects the desired addition ratio from the displayed a plural addition ratios by an addition rate selecting means 4, the picture signals S1 and S2 are added at the selected first addition ratio, a first addition signal S3 is obtained and displayed on the display means 3. The operator confirms the displayed addition picture, and when it is necessary to correct this addition picture, the operator selects a desired addition ratio as the second addition ratio from the table displayed at the display means 3 again, and the picture signals S1 and S2 are added at this addition ratio and a final second picture signal S4 is obtained.