Storage device and storage area management method for reducing garbage collection processing

When a real area is allocated to a write destination virtual area in a virtual volume, a storage device determines whether or not a size of compressed new data is larger than a size of the allocated real area. In a case where the determination result is true, the storage device determines whether or not there is one or more real areas having a size equal to or larger than the size of the compressed new data and including a garbage area that is a real area serving as garbage. In a case where the determination result is true, the storage device selects one or more real areas including the garbage area, allocates, instead of the allocated real area, the selected one or more real areas to the write destination virtual area, and manages the allocated real area as a garbage area.