A method for laying out decoupling cells in a semiconductor integrated circuit including a plurality of paths. The method includes extracting from a timing analysis result a timing slack amount as a timing margin for power supply noise in one of the paths serving as a target path, converting the extracted timing margin to a noise tolerance amount, comparing the noise tolerance amount and a power supply noise amount of the target path, and determining whether or not a decoupling cell must be additionally laid out in the target path based on the comparison result.