An obstacle detection device capable of appropriately setting an area for detecting an obstacle in the vicinity of a machine body of a construction machine in consideration of the state of inclination of the construction machine with respect to its surrounding ground area includes an inclination information acquisition section to obtain inclination information of the machine body with respect to a surrounding ground area in the vicinity of the construction machine, and an area setting section to set a monitoring area for detecting an obstacle, and to modify the monitoring area according to the inclination information.