PURPOSE: To cut off the power source instantaneously and to prevent a form from being overheated by providing a circuit which cuts off power supply to a heat source.

CONSTITUTION: If a CPU runs away, a CPU runaway detecting device detects the runaway of the CPU and stops feeding electricity to a relay coil. Consequently, the relay contact is broken to cut off the power line of the heat source. Further, if a triac for heat source control abnormally operates, a triac abnormality detecting device compares the detection signal of a current detecting device provided on the power line of the heat source with an input signal to the triac and cuts off power supply to the relay coil if a current flows to the power line of the heat source although the input signal to the triac is OFF. Consequently, the relay contact is broken, and the power line of the heat source can be cut off. Thus, the heat source can be turned OFF instantaneously even if the CPU runs away and if a printer abnormally operated.