A state estimation apparatus (1000, 1000A, 2000, 2000A, 2000B) for estimating an internal state of an observation target, the apparatus comprising: a first observation obtaining unit (1, 1A, 11) configured to obtain first observation data from an observable event at predetermined time intervals; and a second observation obtaining unit (2, 2A, 12) configured to obtain second observation data from the observable event at predetermined time intervals; a prior probability distribution prediction unit configured to perform prediction using, as prior probability distribution data at current time t, posterior probability distribution data obtained at preceding time t-1 indicating a probability distribution of an internal state of an observation target to obtain predictive probability distribution data indicating a probability distribution of the internal state of the observation target at the current time t; a first likelihood obtaining unit (3, 31) configured to obtain a first likelihood based on the first observation data; a second likelihood obtaining unit (4, 32) configured to obtain a second likelihood based on the second observation data; characterized by a likelihood combining unit (5, 5A, 5B, 5C) configured to obtain a combined likelihood (wc) by combining the first likelihood and the second likelihood based on first reliability data (P) indicating reliability of the first observation data, and second reliability data (P) indicating reliability of the second observation data according to the following formula wc = P 1 / P 1 + P 2 × w 1 + P 2 / P 1 + P 2 × w 2, wherein wc is the combined likelihood, w1 is the first likelihood, w2 is the second likelihood, P is the first reliability data, and P is the second reliability data; and a posterior probability distribution obtaining unit configured to obtain posterior probability distribution data indicating a probability distribution of the internal state of the observation target at the current time t from the combined likelihood (wc) and the predictive probability distribution data. a first observation obtaining unit (1, 1A, 11) configured to obtain first observation data from an observable event at predetermined time intervals; and a second observation obtaining unit (2, 2A, 12) configured to obtain second observation data from the observable event at predetermined time intervals; a prior probability distribution prediction unit configured to perform prediction using, as prior probability distribution data at current time t, posterior probability distribution data obtained at preceding time t-1 indicating a probability distribution of an internal state of an observation target to obtain predictive probability distribution data indicating a probability distribution of the internal state of the observation target at the current time t; a first likelihood obtaining unit (3, 31) configured to obtain a first likelihood based on the first observation data; a second likelihood obtaining unit (4, 32) configured to obtain a second likelihood based on the second observation data; characterized by a likelihood combining unit (5, 5A, 5B, 5C) configured to obtain a combined likelihood (wc) by combining the first likelihood and the second likelihood based on first reliability data (P) indicating reliability of the first observation data, and second reliability data (P) indicating reliability of the second observation data according to the following formula wc = P 1 / P 1 + P 2 × w 1 + P 2 / P 1 + P 2 × w 2, wherein wc is the combined likelihood, w1 is the first likelihood, w2 is the second likelihood, P is the first reliability data, and P is the second reliability data; and a posterior probability distribution obtaining unit configured to obtain posterior probability distribution data indicating a probability distribution of the internal state of the observation target at the current time t from the combined likelihood (wc) and the predictive probability distribution data.