A programmable controller includes one or more extension units; a CPU unit for controlling the extension units; and a charging element. The CPU unit includes a voltage detection circuit for detecting an output voltage of the charging element and outputting a voltage reduction signal when the detected output voltage is lowered to a level equal to or lower than a reference value and a signal output circuit for outputting a power supply stop signal in response to the voltage reduction signal. Each extension unit includes a voltage detection circuit for detecting an output voltage of an power circuit and outputting a power supply stop signal when the detected output voltage is lowered to a level equal to or lower than a reference value and a power supply stop circuit for stopping the power circuit in response to the power supply stop signal.