PURPOSE: To obtain an inexpensive printer capable of soon detecting the bias of spot light from a printing paper and then of preventing malprinting by using a system in which reflected light from a means of irradiation for the next printing place on a printing paper is used in judging the presence or absence of the printing paper.

CONSTITUTION: To let the next printing place on a printing paper 2 know, a spot light 6 is irradiated from the luminous element 5 of a printer onto the printing paper 2 and the reflected light is received by a light receiving element 7. In such a printer, the level of the light receiving element 7 is detected by a printer controller to judge the presence or absence of the printing paper 2. In short, since reflected light from white printing papers is strong but reflected light from a dark platen 1 is weak in general, the presence of the printing paper 2 is judged from the voltage level of the light receiving element 7 and thereby malprinting on the platen 1 is completely prevented by the printer controller (b).