A microcomputer includes a plurality of operation mode selecting terminals to which data for selecting plural operation modes are set. The plurality of operation mode selecting terminals is designed so as to be usable as general-purpose input terminals or output terminals. A decoder decodes the data set to the plurality of operation mode selecting terminals and outputting a mode signal for switching an internal function in accordance with a selected operation mode. A timing signal output unit outputs to the decoder a timing signal for making the decoder execute a decode operation. The timing signal output unit outputs the timing signal when at least one of power-on-reset and an externally controlled reset is varied from an active state to an inactive state.