PURPOSE: To prevent a misjudgement from being made through a photointerrupter, by disposing a light-receiving part of the photointerrupter at such a wall surface position that the reception of light from a light-emitting part of the photointerrupter to the light-receiving part is not hindered by external light.

CONSTITUTION: Where a recording paper inserting port 20 is slanted along an inserting direction for easy insertion of a printing paper 10, a light-receiving part 21B is disposed at a wall part on the recording paper inserting side of a detecting groove 21D of a photointerrupter 21, namely, on the right side, while a light-emitting part 21A is disposed at a wall part on the left side. Though external light coming in the inserting direction of the recording paper 10, for example, in the direction of an arrow 23A may reach the light-emitting part 21A, the light can not be received by the light-receiving part 21B. In addition, external light penetrating into the inserting port 20 from the left side of an appratus as indicated by an arrow 23B can not reach the light-receiving part 21B,to say nothing of the light-emitting part 21A, because the inserting port 20 is slanted. Accordingly, a malfunction due to reception of external light by the light-receiving part 21B can be prevented from occurring.