To improve snapshot performance and copying performance on a block device level in a capacity of an allocation-on-use volume. Provided is a storage system coupled to a network includes one or more volumes constituted of a plurality of blocks. The plurality of blocks include a first block and a second block, and a physical storage area is allocated to at least one of the plurality of blocks. When it is judged that the physical storage area has not been allocated to the target first block of a received writing request, the storage system writes data to be written by the writing request in the first block without copying data stored in the first block to the second block.