1. A computer aided design apparatus comprising: a simulation unit generating a signal characteristic of a transistor by simulating an operation of the transistor with simulation parameters; a characteristic value extracting unit extracting from the signal characteristic, characteristic values for distinguishing between a first signal characteristic generated by setting first values in the simulation parameter and a second signal characteristic generated by setting second values in the simulation parameters, at least any one of the second values being different from the first values; and a simulation parameter determining unit determining a first mapping relationship to the simulation parameters from the characteristic values with a combination of the characteristic values and set values, the characteristic values being respectively obtained by setting a plurality of combinations of the set values in the simulation parameters.