A congestive placement preventing apparatus for modifying a circuit layout includes an analyzing module, a defining module and an extension module. The analyzing module performs a congestion analysis on the circuit layout to generate an analysis result. The defining module defines a congestion region and a share region adjacent to the congestion region on the circuit layout according to the analysis result. A density of electronic cells of the congestion region is higher than that of electronic cells of the share region. The extension module arranges a plurality of electronic cells in the congestion region to the congestion region and the share region, thereby reducing the density of electronic cells in the congestion region.