A parameter copying method is applied to a duplex system in which MPU and a main memory are duplicated and duplex operations on a hot standby system are performed. The parameter copying method includes cache reading data in the main memory corresponding to one MPU, cache writing the data read in the cache reading step on an as-is basis, and writing the data into the main memory corresponding to the one MPU by a block write that is produced by a cache replace caused due to the cache writing step, and also writing the same data into the main memory corresponding to the other MPU by the block write on a basis of a mirrored write.