specifically cpu 321 instructs heater controller 400 that the print mode is entered in heater controller 400 pulse signal generation circuit 410 operates in response to the instruction issued from cpu 321 to adjust the pulse signal 39 s duty ratio so as to set a voltage level corresponding to the maximum temperature for example of 200 u00b0 c and output the adjusted pulse signal and when comparator 20 outputs detection signal dta of u201c 1 u201d heater controller 400 turns off heater lamp 4 to stop the maximum temperature from further increasing subsequently the prescribed reference voltage is set to a voltage level corresponding to a minimum temperature for example of 180 u00b0 c in the print mode more specifically pulse signal generation circuit 410 operates in response to instruction issued from cpu 321 to adjust the pulse signal 39 s duty ratio so as to set a voltage level corresponding to the minimum temperature for example of 180 u00b0 c and output the adjusted pulse signal and when comparator 20 outputs detection signal dta of u201c 0 u201d heater controller 400 turns on heater lamp 4 to stop the minimum temperature from further decreasing thus switching the prescribed reference voltage in level and controlling based on a decision operation as described above can repeatedly be performed so that hot roller 3 in the print mode can be held constantly within the range of the maximum temperature for example of 200 u00b0 c to the minimum temperature for example of 180 u00b0 c in the present embodiment temperature detector 19 can adjust the.