A PROM built-in micro computer has a semiconducting nonvolatile memory which can be written into electrically, and has a micro computer. It detects a semiconducting nonvolatile memory cell whose readout current is less than or equal to a constant value at a time of data comparison after a writing operation. The PROM built-in micro computer is adapted to prevent a readout error which may result from a fluctuation of temperature or supply voltage. This may be done by changing a reference potential at the time of a data comparison, by changing the sensitivity of a sense amplifier, or by changing a readout current or a threshold value of the PROM cell.