1. An object detection device, comprising: an acquisition unit configured to acquire information indicating a temperature distribution within a detection range; a storage unit configured to store background information indicating a temperature distribution within the detection range when no target object exists in the detection range; a detection unit configured to detect existence or absence of a target object in the detection range, based on a change of the acquired information with respect to the background information; and an update unit configured to repeatedly update the background information stored in the storage unit, wherein the update unit divides the background information into a detection region including a region where the target object is detected by the detection unit and a non-detection region including a region other than the detection region, and the update unit performs, with respect to the non-detection region, a first background updating process for the update of the background information based on the acquired information and performs, with respect to the detection region, a second background updating process for the update of the background information using a correction value found from a variation of a temperature in an extraction region including at least a portion of the non-detection region, the variation of the temperature being calculated in the first background updating process.