An information processing device that tracks a predetermined target in a video made of a plurality of frames, comprising: a particle initial setting means that sets at least one particle on the target in an image of a previously-selected frame in the video; a frame acquiring means that sequentially acquires temporally-previous frames and temporally-next frames in the video after the previously-selected frame; a tracking range's outer edge determining means that determines, as an outer edge of a particle tracking range, a part corresponding to the target in an image of the previous frame where a degree of difference in pixel values between the image of the previous frame and an image of the next frame acquired by the frame acquiring means is greater than or equal to a threshold; a particle setting means that sets particles in the image of the next frame at positions of pixels which are within a predetermined range with reference to positions of particles in the image of the previous frame and only within the outer edge determined by the tracking range's outer edge determining means so as to avoid that particles scatter out of the border of the target and which have color information similar to color information on pixels at the positions of the reference particles; a target specifying means that specifies a position of the target in the image of the next frame from the positions of the particles set by the particle setting means; a track determining means that determines whether the target can be tracked based on the particles set by the particle setting means; a particle resetting means that, when the track determining means determines that tracking is impossible, resets particles in the image of the next frame according to the positions of the particles set by the particle initial setting means or the particle setting means; and a storing means that stores the positions of the particles set by the particle initial setting means or the particle setting means, wherein when the tracking range's outer edge determining means cannot determine an outer edge of the tracking range, the particle setting means sets particles in the image of the next frame according to the positions of the particles in the previous frame for which the outer edge of the tracking range can be determined with reference to the storing means. a particle initial setting means that sets at least one particle on the target in an image of a previously-selected frame in the video; a frame acquiring means that sequentially acquires temporally-previous frames and temporally-next frames in the video after the previously-selected frame; a tracking range's outer edge determining means that determines, as an outer edge of a particle tracking range, a part corresponding to the target in an image of the previous frame where a degree of difference in pixel values between the image of the previous frame and an image of the next frame acquired by the frame acquiring means is greater than or equal to a threshold; a particle setting means that sets particles in the image of the next frame at positions of pixels which are within a predetermined range with reference to positions of particles in the image of the previous frame and only within the outer edge determined by the tracking range's outer edge determining means so as to avoid that particles scatter out of the border of the target and which have color information similar to color information on pixels at the positions of the reference particles; a target specifying means that specifies a position of the target in the image of the next frame from the positions of the particles set by the particle setting means; a track determining means that determines whether the target can be tracked based on the particles set by the particle setting means; a particle resetting means that, when the track determining means determines that tracking is impossible, resets particles in the image of the next frame according to the positions of the particles set by the particle initial setting means or the particle setting means; and a storing means that stores the positions of the particles set by the particle initial setting means or the particle setting means, wherein when the tracking range's outer edge determining means cannot determine an outer edge of the tracking range, the particle setting means sets particles in the image of the next frame according to the positions of the particles in the previous frame for which the outer edge of the tracking range can be determined with reference to the storing means.