1. A state estimating apparatus comprising: a first element which measures a value of a first state variable indicative of a state of an object, disperses at least some particles among a plurality of particles around previous positions in a state variable space defined by a second state variable, transits each of the plurality of particles to a current position according to a current model, said current model being unique to each particle and fluidly changing, among a plurality of models, assesses a likelihood of each particle relative to a current measured value of the first state variable, and estimates a current state of the object on the basis of the likelihood; and a second element which preferentially allows the particle, which has been assessed by the first element and the likelihood of which is high, to remain or increase, and preferentially extinguishes the particle, which has been assessed by the first element and the likelihood of which is low, and then determines a next model unique to each particle on the basis of the current model unique to each particle.