To provide a technology for supporting the design of a trimming allowance by which generation of abnormal trimming caused by too small trimming allowance and the lowering of yield caused by too large trimming allowance of steel sheets which are produced through processing in a plurality of manufacturing processes in a steel sheet manufacturing works are minimized.

By an actual sheet width data collecting and accumulating part 101, the actual sheet width data about the steel sheets manufactured by the trimming allowance specified so far are collected and accumulated. By an actual sheet width margin value-calculating part 102, a sheet width margin value which is a margin value of the actual sheet width corresponding to the generation of the abnormal trimming is calculated. A minimum value of the sheet width margin which is the minimum value of the sheet width margin value of the steel sheet of a production schedule is predicted by a predicting part 104 of the minimum value of the sheet width from the production schedule input by an input part 103. By a trimming allowance deciding part 105, the designed value of the trimming allowance of the steel sheet of the production schedule is decided and the predicted minimum value of the sheet width margin and decided designed value of the trimming allowance are output by an output part 106.