A microcomputer comprises a CPU; a nonvolatile memory; a plurality of volatile memories; a system bus; a program transfer bus; a program transfer section; an address conversion section; and a voltage detection section. The volatile memories include a plurality of memories switchable to be used as transfer and execution memories in accordance with a program execution state by the CPU. Where the voltage detected by the voltage detection section is lower than a first voltage, the program transfer section transfers the part of the program stored in the nonvolatile memory to the transfer memory, and the address conversion section converts an address in the nonvolatile memory output from the CPU into an address in the execution memory.