A semiconductor device comprising: a) a cache configured to store auxiliary information for use in reading compressed data, the auxiliary information being stored in a memory together with the compressed data, and wherein the auxiliary information is data size information indicating a data length of compressed data; b) a control unit (32, 334, 336) which is configured to, when receiving a read request for reading the compressed data stored in the memory, read from the cache auxiliary information about the compressed data if the auxiliary information is stored in the cache, or read the auxiliary information about the compressed data from the memory and store the auxiliary information about the compressed data in the cache if the auxiliary information is not stored in the cache, wherein the read request includes the data address before compression of the desired compressed data and the data length before compression of the desired compressed data, wherein said control unit (32, 334, 336) is configured to read the compressed data from the memory using the auxiliary information about the compressed data and the data address included in the request; and c) an expansion unit configured to expand the compressed data read from the memory; wherein the data address before compression of the desired compressed data is the same as the address at which the desired compressed data is stored in the memory, wherein, when reading the auxiliary information about the compressed data from the memory and storing the auxiliary information about the compressed data in the cache, the control unit (32, 334, 336) is configured to read auxiliary information of a predetermined unit for reading and store in the cache the auxiliary information of the predetermined unit for reading, the auxiliary information of the predetermined unit for reading including the auxiliary information about the compressed data, and wherein the control unit (32, 334, 336) is configured to, when reading the auxiliary information of the predetermined unit for reading from the memory and storing the auxiliary information in the cache, also read, as advance reading, auxiliary information of a next predetermined unit for reading and store the auxiliary information of the next predetermined unit for reading in the cache, the auxiliary information of the predetermined unit for reading and the auxiliary information of the next predetermined unit for reading being contiguous in address. a) a cache configured to store auxiliary information for use in reading compressed data, the auxiliary information being stored in a memory together with the compressed data, and wherein the auxiliary information is data size information indicating a data length of compressed data; b) a control unit (32, 334, 336) which is configured to, when receiving a read request for reading the compressed data stored in the memory, read from the cache auxiliary information about the compressed data if the auxiliary information is stored in the cache, or read the auxiliary information about the compressed data from the memory and store the auxiliary information about the compressed data in the cache if the auxiliary information is not stored in the cache, wherein the read request includes the data address before compression of the desired compressed data and the data length before compression of the desired compressed data, wherein said control unit (32, 334, 336) is configured to read the compressed data from the memory using the auxiliary information about the compressed data and the data address included in the request; and c) an expansion unit configured to expand the compressed data read from the memory; wherein the data address before compression of the desired compressed data is the same as the address at which the desired compressed data is stored in the memory, wherein, when reading the auxiliary information about the compressed data from the memory and storing the auxiliary information about the compressed data in the cache, the control unit (32, 334, 336) is configured to read auxiliary information of a predetermined unit for reading and store in the cache the auxiliary information of the predetermined unit for reading, the auxiliary information of the predetermined unit for reading including the auxiliary information about the compressed data, and wherein the control unit (32, 334, 336) is configured to, when reading the auxiliary information of the predetermined unit for reading from the memory and storing the auxiliary information in the cache, also read, as advance reading, auxiliary information of a next predetermined unit for reading and store the auxiliary information of the next predetermined unit for reading in the cache, the auxiliary information of the predetermined unit for reading and the auxiliary information of the next predetermined unit for reading being contiguous in address.