By an electronic control unit, an area allocation method, or a computer-readable non-transitory storage medium storing an area allocation program, an area securement request is accepted from an area securement request application, it is determined whether a capacity of an unallocated area of a storage exceeds a request capacity, a part of unused area of an allocated area already allocated to an area-allocated application is release as a release area, a total capacity of at least a part of the capacity of the unallocated area and a capacity of the release area is allocated as the allocated area.