For the purpose of readily specifying a portion of the circuit which has a high possibility of error occurring due to a variation in the supply voltage so that the specified vulnerable portion is countermeasured in a mask layout process, a simulation section simulates the operation of a semiconductor integrated circuit to obtain a transition timing of an input signal that is input to each circuit element. A simultaneous-operation circuit element number detecting section detects, based on a result of the simulation, the number of circuit elements which are supplied with the supply voltage through a common power supply line and in which transition timings of input signals occur within a predetermined time interval (e.g., 0.3 ns or shorter). A supply voltage variation level estimating section estimates the variation level of the supply voltage according to the number of circuit elements which is detected by the simultaneous-operation circuit element number detecting section.