An analog-digital converting device is designed such that when an operation of converting a special analog signal into a digital signal and an operation of converting another analog signal into a digital signal are instructed concurrently, priority is given to the conversion operation of the special analog signal, The analog-digital converting device includes an analog multiplexer having a main channel and a sub channel, a sample holder, an AD converter, a conversion result register having a plurality of storage areas, and an AD control circuit for controlling the drive of the individual components according to an instruction of a CPU. When the analog signals input to the channels are converted into digital signals in sequence, priority is given to AD conversion of the analog signal input to the main channel over AD conversion of the analog signal input to the sub channel.