A programmable controller includes a CPU unit, a communication unit and peripheral units connected together through an internal bus. The communication unit has a bus master function, including a cache memory for recording IO data stored in the memory of an input-output unit. When a message is received, it is judged whether the IO data stored in the memory of the input-output unit specified by this message is updated or not. If the data are not updated, a response is created based on the IO data stored in the IO data stored in the cache memory. If the data are updated, the input-output unit is accessed and updated IO data are obtained and a response is created based on the obtained IO data.