A safety controller of building block type has an array of a plurality of IO units each including IO circuits and a single IO unifying unit connectable to these IO units. The IO unifying unit has an IO power-receiving terminal for receiving power from an external IO power source and an IO power distributing terminal for distributing the received power to the array of IO units after passing inside the IO unifying unit. The array of IO units includes distribution mains extending along the array and having a starting end connected to the IO power distributing terminal and distribution branch lines for supplying power to each of the IO circuits inside the IO units. An IO power shutoff circuit is provided to the IO unifying unit on upstream side of the IO power distributing terminal and to each of the distribution branch lines in each of the IO units.