To provide a propagation delay controlling method which can suppress variation in manufacturing due to changes in temperature and voltage, and can prevent man-hour and a circuit area from increasing.

The propagation delay controlling method comprises the steps of preparing in advance a plurality of time constant control libraries each having a different time constant, creating a net list in which an initial time constant control library having the minimum time constant in the plurality of time constant control libraries is inserted into each signal line, creating a delay information file including the wiring delay information of each signal line after arrangement of cells and wiring are finished, judging if delay control between respective signal lines is needed or not based on the delay information file, selecting the time constant control library which reduces the time constant difference between the respective signal lines from the plurality of time constant control libraries prepared in advance if the delay control is needed, and replacing the initial time constant control library with the selected time constant control library.