PURPOSE: To carry out an exact judgment by judging the existence of a paper sheet based on a value detected by an optical sensor element.

CONSTITUTION: When a power switch is turned off with a printing paper of high quality kept on a paper conveyance route, for example, a control program stored in the program memory range 28 of a ROM 28 starts. A reflection type photo sensor 20 converts a current value according to the reflected light into voltage to be detected, and the detected value is inputted in a CPU 24. The CPU 24 synchronizes with a fixed cycle or paper feed to read out the output value (referred to as the output of a photo sensor 20). The CPU compares the input value with the first reference value 28 which is stored in the first reference value of a ROM. Since a printing paper of superior quality has high light reflectance, the input value becomes larger than the first reference line for taking a paper existence process. It is, thus possible to judge the existence of paper exactly.