1. An abnormality detection system comprises: generation distribution storage means that stores a generation distribution of features of predetermined data estimated to be input, or generates, on the basis of actually input data, a generation distribution of the features of the input data; reference data generation means that generates reference data indicative of normal data from a data sequence of data to be input, or beforehand stores predetermined data as the reference data; dissimilarity degree calculation means that obtains, every piece of the data sequence, a probability that, when features are virtually generated from the generation distribution, the features are nearer to the reference data than the features of each piece of the data, and takes the probability as a dissimilarity degree between each piece of the data and the reference data; and abnormality determination means that performs the determination of abnormal data on the basis of the dissimilarity degree.