A state estimation apparatus obtains observation data sets for a tracking target object, and estimates an internal state of the object using likelihoods obtained from the observation data sets and the reliability of each observation data set. A first observation obtaining unit obtains first observation data. A second observation obtaining unit obtains second observation data. A first likelihood obtaining unit obtains a first likelihood based on the first observation data. A second likelihood obtaining unit obtains a second likelihood based on the second observation data. A likelihood combining unit obtains a combined likelihood based on the first and second likelihoods, first reliability data indicating the reliability of the first observation data, and second reliability data indicating the reliability of the second observation data. A posterior probability distribution obtaining unit obtains posterior probability distribution data indicating a probability distribution of the internal state of the observation target at current time t from the combined likelihood and predictive probability distribution data.