A backup system, comprising: (a) a data dividing means (11, 111) for dividing backup data into a plurality of block data; (b) a block detecting means (12, 112) for checking whether the respective block data obtained by division by the data dividing means are already stored in a storage device; and (c) a data writing means (13, 113) for storing the respective block data obtained by division by the data dividing means into the storage device and, when storing other block data of a same content as block data already stored in the storage device, causing to refer to the block data already stored in the storage device as the other block data, characterised in that: (d) the block detecting means (12, 112) is configured to, in a case of having detected that specific block data of the block data obtained by division by the data dividing means has already been stored in the storage device, detect a share rate between a block set configured of a plurality of block data following the specific block data in the backup data and a block set configured of a plurality of block data following the block data of the same content as the specific block data already stored in the storage device; and (e) the data writing means (13, 113) is configured to, in case that the share rate detected by the block detecting means is equal to or less than a previously set value, newly store the specific block data into the storage device. (a) a data dividing means (11, 111) for dividing backup data into a plurality of block data; (b) a block detecting means (12, 112) for checking whether the respective block data obtained by division by the data dividing means are already stored in a storage device; and (c) a data writing means (13, 113) for storing the respective block data obtained by division by the data dividing means into the storage device and, when storing other block data of a same content as block data already stored in the storage device, causing to refer to the block data already stored in the storage device as the other block data, characterised in that: (d) the block detecting means (12, 112) is configured to, in a case of having detected that specific block data of the block data obtained by division by the data dividing means has already been stored in the storage device, detect a share rate between a block set configured of a plurality of block data following the specific block data in the backup data and a block set configured of a plurality of block data following the block data of the same content as the specific block data already stored in the storage device; and (e) the data writing means (13, 113) is configured to, in case that the share rate detected by the block detecting means is equal to or less than a previously set value, newly store the specific block data into the storage device. (d) the block detecting means (12, 112) is configured to, in a case of having detected that specific block data of the block data obtained by division by the data dividing means has already been stored in the storage device, detect a share rate between a block set configured of a plurality of block data following the specific block data in the backup data and a block set configured of a plurality of block data following the block data of the same content as the specific block data already stored in the storage device; and (e) the data writing means (13, 113) is configured to, in case that the share rate detected by the block detecting means is equal to or less than a previously set value, newly store the specific block data into the storage device.