A simulator for accurately simulating a deterioration amount and a recovery amount of transistor characteristics, by which a semiconductor device can be designed with high reliability, and the method are provided. When a gate voltage of a negative level (a negative bias voltage) “Vg” is applied to a gate of the transistor, characteristics of the transistor are deteriorated. When application of the negative level gate voltage “Vg” is terminated (when applying a bias free voltage), the deteriorated transistor characteristics are recovered. In a deterioration period and a recovery period, a logarithm “log(t)” is obtained for an application time “t” of the gate voltage, a deterioration amount ΔPD(t)=CD+BD·log(t) is calculated by using constants CD and BD depending on the negative bias voltage, a recovery amount ΔPR(t)=CR+BR·log(t) is calculated by using constants CR and BR depending on the bias free voltage, and the deterioration amount (ΔPD), the recovery amount (ΔPR) and a basic deterioration amount (XD) are summed up. Preferably, passage of time is divided, and a deterioration amount and a recovery amount are obtained for each time zone by using different deterioration and recovery functions for each time zone.