An input processing unit of safe digital controller includes units for determining ground when a voltage V from an input terminal is equal to or lower than a ground determination value; for outputting a first specified value regardless of the voltage V during a first given period immediately after the controller starts; for outputting a value based on the voltage V when the voltage V is equal to or higher than the ground fault determination value and equal to or lower than a disconnection determination value after the first given period, outputting a third specified value during a second given period after the first given period when the voltage V is lower than the ground fault determination value, and outputting the first specified value when a state in which the voltage V is lower than the ground fault determination value is maintained for the second given period or longer; and for outputting a second specified value during the second given period when the voltage V is higher than the disconnection determination value after the first given period, and outputting the first specified value when a state in which the voltage V is higher than the disconnection determination value is maintained for the second given period or longer.