1. A discriminant function specifying device, comprising: a setting part configured to calculate a discriminant function by machine learning using a learning input parameter, the discriminant function being for separating a distribution of a genuine similarity from a distribution of impostor similarity, the learning input parameter being set before executing the machine learning, and to set a plurality of discriminant functions, each of the plurality of discriminant functions being obtained by changing values of the learning input parameter and a learning output parameter, the learning output parameter being a parameter of the calculated discriminant function; a calculation part configured to calculate an error rate for each of the plurality of discriminant functions set by the setting part; and an extraction part configured to extract one discriminant function among the plurality of discriminant functions, the one discriminant function having the error rate calculated by the calculation part which meets a predetermined error condition.