PURPOSE: To prevent malfunction by a thermal printer due to the disorderly run of software by providing a status decision means which validates a print ON signal from a controller only if all sensors are in a printing possible state.

CONSTITUTION: If a printing head 4 is lowered, a sensor S1 become 'LOW' to allow detection of a slit 14a on a disc 14 by a sensor S2. Then the S2 becomes 'HIGH' to allow output of 'LOW' signal to a thermal head driver circuit 8. After rotation of a transfer drum 5 to a transfer start position B, the state of 'white' of paper and 'black' of the transfer drum 5 is determined by a paper presence sensor 21. If the paper is present, the 'LOW' signal is transmitted to a controller 6. A paper detection microswitch 26 connects the 'LOW' signal to the controller 6 when a cover is closed. Consequently, the transfer is made possible by the sensors only when a cover opening sensor 26 is 'LOW', the sensor S1 is 'LOW', the sensor S2 is 'HIGH' and the paper presence sensor 21 is 'LOW'.