A stored program controlled switching system is comprised of a first set of detachably mounted nonvolatile semiconductor memory devices such as memory cards for respectively storing data of different types including a control program, a second set of detachably mounted nonvolatile semiconductor memory devices for respectively storing the data of different types. A first controller is provided for operating the first set of memory devices in a working mode and the second set of memory devices in a standby mode, copying data from the memory devices of the working mode into a main memory and further into the memory devices of the standby mode, and performing a switchover between the operating modes when trouble occurs in the first set of memory devices. A second controller operates the switching system according to the data stored in the main memory.