A suspicious person detection device comprising: a person detection unit capable of detecting a position of a person on a per-person basis; a person tracking unit arranged to continuously track the person's position detected by said person detection unit; a suspicious person detection unit arranged to detect a suspicious person on the basis of a tracking result of said person tracking unit, wherein said person tracking unit is arranged to determine, as an already-authenticated person, a person authenticated when coinciding with a predetermined first condition, the predetermined first condition being that a person, who unlocks a predetermined door, is determined to be the already-authenticated person, said person tracking unit being further arranged to determine, as an accompanying person, a person who coincides with a predetermined second condition related to the already-authenticated person when authenticated, wherein the predetermined second condition is that the distance from the already-authenticated person is smaller than a predetermined second distance, and said suspicious person detection unit is arranged to issue an alarm when the accompanying person is distanced at a predetermined first distance or greater from the already-authenticated person; and a distance change unit arranged to obtain the difference D between the magnitude of a first velocity vector of the already-authenticated person and the magnitude of a second velocity vector of the accompanying person, and an angular difference theta between said first and second velocity vectors, and to set the predetermined first distance based on the obtained difference D and angular difference theta. a person detection unit capable of detecting a position of a person on a per-person basis; a person tracking unit arranged to continuously track the person's position detected by said person detection unit; a suspicious person detection unit arranged to detect a suspicious person on the basis of a tracking result of said person tracking unit, wherein said person tracking unit is arranged to determine, as an already-authenticated person, a person authenticated when coinciding with a predetermined first condition, the predetermined first condition being that a person, who unlocks a predetermined door, is determined to be the already-authenticated person, said person tracking unit being further arranged to determine, as an accompanying person, a person who coincides with a predetermined second condition related to the already-authenticated person when authenticated, wherein the predetermined second condition is that the distance from the already-authenticated person is smaller than a predetermined second distance, and said suspicious person detection unit is arranged to issue an alarm when the accompanying person is distanced at a predetermined first distance or greater from the already-authenticated person; and a distance change unit arranged to obtain the difference D between the magnitude of a first velocity vector of the already-authenticated person and the magnitude of a second velocity vector of the accompanying person, and an angular difference theta between said first and second velocity vectors, and to set the predetermined first distance based on the obtained difference D and angular difference theta.