1. A model parameter extracting apparatus comprising: a binning processor configured to carry out a binning process for limiting an effective range of a semiconductor device model used in a circuit simulation; and a model parameter extractor configured to extract a model parameter used in the circuit simulation, for each of a plurality of bins formed by the binning process, wherein the model parameter extractor selects one of the plurality of bins as a target bin, the model parameter extractor extracts a first model parameter for predicting an electric characteristic of a semiconductor device at a first end portion of the target bin, based on the first model parameter, the model parameter extractor sets up a candidate for a second model parameter for predicting an electric characteristic of the semiconductor device at a second end portion of the target bin, based on the first model parameter and the candidate for the second model parameter, the model parameter extractor calculates a limited curve representing the electric characteristic of the semiconductor device, the model parameter extractor finds a start-point-side gradient representing a gradient of a tangent line of the limited curve including a start point; and an end-point-side gradient representing a gradient of a tangent line of the limited curve including an end point, and based on a result of a comparison between the start-point-side gradient and the end-point-side gradient, the model parameter extractor extracts the second model parameter.