A target tracking system which tracks a target using a plurality of virtual particles, comprising: a detector which is adapted to detect the targets based on a signal wave from the targets and outputs detected results as detected target positions; two or more estimation units, each containing a prediction part which is adapted to predict target positions at a predetermined future timing as predicted target positions using a plurality of virtual particles for the detected target positions detected by the detector; and an estimation part which is adapted to estimate true target positions at present as estimated target positions based on the detected target positions from the detector and the predicted target positions predicted by the prediction part; and a fluctuation distribution unit which is adapted to generate fluctuations of each motion parameter such as an initial position, a speed and an acceleration of a target arising from at least disturbances and measurement errors, and to output the generated fluctuations to the two or more estimation units, wherein the estimation part is adapted to calculate a likelihood that a predicted target position is a true target position, and to make the average value of a plurality of predicted target positions which have been weighted using the likelihood; and wherein the prediction part receives fluctuations from the fluctuation distribution unit and the estimated target positions from the estimation part, generates a movement model in which the fluctuations received from the fluctuation distribution unit are taken into the motion parameters stored in advance, and calculates the predicted target positions by applying the movement model to the estimated target positions using a plurality of virtual particles. a detector which is adapted to detect the targets based on a signal wave from the targets and outputs detected results as detected target positions; two or more estimation units, each containing a prediction part which is adapted to predict target positions at a predetermined future timing as predicted target positions using a plurality of virtual particles for the detected target positions detected by the detector; and an estimation part which is adapted to estimate true target positions at present as estimated target positions based on the detected target positions from the detector and the predicted target positions predicted by the prediction part; and a fluctuation distribution unit which is adapted to generate fluctuations of each motion parameter such as an initial position, a speed and an acceleration of a target arising from at least disturbances and measurement errors, and to output the generated fluctuations to the two or more estimation units, wherein the estimation part is adapted to calculate a likelihood that a predicted target position is a true target position, and to make the average value of a plurality of predicted target positions which have been weighted using the likelihood; and wherein the prediction part receives fluctuations from the fluctuation distribution unit and the estimated target positions from the estimation part, generates a movement model in which the fluctuations received from the fluctuation distribution unit are taken into the motion parameters stored in advance, and calculates the predicted target positions by applying the movement model to the estimated target positions using a plurality of virtual particles.