A collision detector accurately detects a situation where a subject vehicle is likely to come into contact with an external object. The collision detector defines a subject area including an enter-determination area and an exist-determination area. The enter-determination area is defined to determine that the external object entering therein is likely to come into contact with the subject vehicle. The exist-determination area is defined to determine that the external object existing therein is likely to come into contact with the subject vehicle. The detector determines whether the external object comes into the enter-determination area or the external object exists in the exist-determination area. The detector outputs that the subject vehicle is likely to come into contact with the external object when the external object comes into the enter-determination area or when the external object exists in the exist-determination area.