In an apparatus for detecting turn-on state of plural heaters including an electric power source for supplying electric power to the plural heaters, a controller controls the electric power source to be turned on and off so that each of the plural heaters is turned on and off in a predetermined manner, and a current sensor measures the total current supplied from the electric power source to the plural heaters. Then, a comparator compares the total current measured by the current sensor with plural reference values different from each other and outputting signals for representing comparison results thereof, and a judging circuit judges turn-on or off state of each of the plural heaters based on the signals outputted from the comparators.