1. A state output probability calculating method for a mixture distribution HMM, executed by a pattern recognition apparatus which performs pattern recognition of an observation signal by using a mixture distribution HMM in which an event of a recognition target stored in a memory is modeled, comprising: a first output probability calculating step of calculating weighted output probabilities of all distributions belonging to a state for a first observation signal, obtaining a maximum value of the weighted output probabilities and a maximum distribution which outputs the maximum value, and defining the obtained maximum value as an output probability of a mixture distribution; a maximum distribution storage step of storing, in the memory, information which specifies a maximum distribution obtained in the first output probability calculating step; and a second output probability calculating step of, when a predetermined condition is satisfied, calculating a weighted output probability of the maximum distribution stored in the memory in the maximum distribution storage step, instead of the first output probability calculating step, for a second observation signal observed after the first observation signal, and defining the calculated weighted output probability as the output probability of the mixture distribution.