1. A pitch estimation apparatus for estimating a fundamental frequency of an audio signal from a fundamental frequency probability density function by modeling the audio signal as a weighted mixture of a plurality of tone models corresponding respectively to harmonic structures of individual fundamental frequencies, so that the fundamental frequency probability density function of the audio signal is given as a distribution of respective weights of the plurality of the tone models, the pitch estimation apparatus comprising: a plurality of function estimators, each being provided with the audio signal, and each estimating the fundamental frequency probability density function by repeating a weight calculation process and an estimated shape specification process, wherein the weight calculation process calculates a weight of each tone model of each fundamental frequency based on an estimated shape of each tone model of each fundamental frequency, the estimated shape indicating a degree of dominancy of a corresponding tone model in a total harmonic structure of the audio signal, and the estimated shape specification process specifies each estimated shape of each tone model of each fundamental frequency based on an amplitude spectrum of the audio signal, the harmonic structure of each tone model of each fundamental frequency, and the weight of each tone model of each fundamental frequency; wherein each function estimator comprises: a similarity analysis part that calculates a similarity index value indicating a degree of similarity between each tone model of each fundamental frequency and each estimated shape specified from the corresponding tone model by the estimated shape specification process; and a weight correction part that reduces a weight of at least one tone model of a certain fundamental frequency having the similarity index value indicating that said one tone model and the corresponding estimated shape are not similar to each other, relative to weights of other tone models having similarity index values indicating that these tone models and corresponding estimated shapes are similar, the pitch estimation apparatus further comprising: a pitch specifying part that receives a sum of the fundamental frequency probability density functions outputted from the plurality of the function estimators and that specifies, as one or more pitches of the audio signal, one or more of the fundamental frequencies corresponding to salient peaks appearing in the sum of the fundamental frequency probability density functions.