A method includes placing standard cells based on a standard cell library and generating layout data, and placing a filler cell selected from among a first type filler cell and a second type filler cell by using the layout data. The filler cell is placed based on a density of a pattern formed in the standard cell. The standard cell library includes data defining the first and second type filler cells. A density of a contact formed on an active region of the second type filler cell to contact the active region of the second type filler cell is lower than a density of a contact formed on an active region of a first type filler cell to contact the active region of the first type filler cell.