A mounted object detection device includes: a load sensor in which a plurality of sensor parts each configured to detect a load are disposed in a matrix shape; and a controller configured to detect a situation of a mounted object on the load sensor on the basis of an output from the load sensor. The controller: detects a load distribution on a detection region of the load sensor on the basis of an output from the load sensor; discerns which of a person or animal and a thing the mounted object is, on the basis of the load distribution; and when having discerned that the mounted object is a thing, discerns whether or not a person or an animal is mounted on the thing, on the basis of temporal change in the load distribution.