A sound analysis apparatus for analyzing an input audio signal based on a weighted mixture of a plurality of tone models which represent harmonic structures of sound sources and which correspond to probability density functions of various fundamental frequencies, the apparatus comprising: a probability density estimation part that sequentially updates and optimizes respective weights of the plurality of the tone models, so that a mixed distribution of frequencies obtained by the weighted mixture of the plurality of the tone models corresponding respectively to the various fundamental frequencies approximates an actual distribution of frequency components of the input audio signal, and that estimates the optimized weights of the tone models to be a fundamental frequency probability density function of the various fundamental frequencies corresponding to the sound sources; and a fundamental frequency determination part that determines an actual fundamental frequency of the input audio signal based on the fundamental frequency probability density function estimated by the probability density estimation part, wherein the probability density estimation part comprises: a storage part that stores data defining sounds that can be generated by sound sources of the input audio signal, and sound source structure data defining a constraint on the sounds that can be simultaneously generated by the sound sources; a form estimation part that creates a plurality of form data by referring to the data defining sounds that can be generated by the sound source during the sequential updating and optimizing of the weights of the tone models corresponding to the various fundamental frequencies, each form data specifying a weight that becomes a peak, a fundamental frequency corresponding to the weight becoming the peak and a sound that is to be generated by the sound sources and that corresponds to the fundamental frequency, the form estimation part further selecting one or more of the form data that does not violate the constraint defined by the sound source structure data, from the plurality of the created form data; and a previous distribution imparting part that imparts a previous distribution to the weights of the tone models corresponding to the various fundamental frequencies so as to emphasize weights corresponding to the fundamental frequencies specified by the form data selected by the form estimation part. a probability density estimation part that sequentially updates and optimizes respective weights of the plurality of the tone models, so that a mixed distribution of frequencies obtained by the weighted mixture of the plurality of the tone models corresponding respectively to the various fundamental frequencies approximates an actual distribution of frequency components of the input audio signal, and that estimates the optimized weights of the tone models to be a fundamental frequency probability density function of the various fundamental frequencies corresponding to the sound sources; and a fundamental frequency determination part that determines an actual fundamental frequency of the input audio signal based on the fundamental frequency probability density function estimated by the probability density estimation part, wherein a storage part that stores data defining sounds that can be generated by sound sources of the input audio signal, and sound source structure data defining a constraint on the sounds that can be simultaneously generated by the sound sources; a form estimation part that creates a plurality of form data by referring to the data defining sounds that can be generated by the sound source during the sequential updating and optimizing of the weights of the tone models corresponding to the various fundamental frequencies, each form data specifying a weight that becomes a peak, a fundamental frequency corresponding to the weight becoming the peak and a sound that is to be generated by the sound sources and that corresponds to the fundamental frequency, the form estimation part further selecting one or more of the form data that does not violate the constraint defined by the sound source structure data, from the plurality of the created form data; and a previous distribution imparting part that imparts a previous distribution to the weights of the tone models corresponding to the various fundamental frequencies so as to emphasize weights corresponding to the fundamental frequencies specified by the form data selected by the form estimation part.