1. A simulation model for designing a semiconductor device being used for a simulation apparatus for designing a semiconductor device, comprising: assuming that, with respect to at least one semiconductor element having at least two first and second electrodes, a transient density of a carrier at a predetermined time t i including a delay of the carrier running between the first and second electrodes is q(t i ), a transient density of the carrier at a predetermined time t i-1 before the time t i is q(t i-1 ), a density of the carrier between the first and second electrodes in a quasi-static state assumed at the time t i is Q(t i ), further a time required for the carrier to reach the other electrode from one of the first and second electrodes is τ, and said q(t i ), q(t i-1 ), Q(t i ), and τ satisfy the following first equation for a time (t i −t i-1 ) from the time t i-1 until the time t i : q(t i )=q(t i-1 )+(t i −t i-1 )/τ[Q(t i )−q(t i-1 )], and these processes are processed by a first assuming unit included in the simulation apparatus; assuming that a current flowing between the first and second electrodes at the time t i is I(t i ), a quasi-static conduction current between the first and second electrodes, determined by the voltage applied at the time t i , is I DC , further a displacement current flowing between the first and second electrodes is dq(t i )/dt, and said I(t i ), I DC , and dq(t i )/dt satisfy the following second equation at the time t i : I(t i )=I DC +dq(t i )/dt, and these processes are processed by a second assuming unit included in the simulation apparatus; and substituting said q(t i ) obtained from the first equation into the second equation to thereby describe said Q(t i ) as a non-quasi-static model, and this process is processed by a non-quasi-static model describing unit included in the simulation apparatus.