PURPOSE: To automate an operation to enhance an operating efficiency and to prevent a malfunction due to misoperation, by a method wherein, the thickness of a printing paper is detected by a paper thickness sensor, and the size of the printing paper is detected by a reflector.

CONSTITUTION: At the time when a carriage is retracted on a home position, a paper thickness detecting device 17 is driven, a paper thickness sensor 8 is pressed, and a reference electric output E1 is detected and written in a RAM. Thereafter, a reflectance on a platen surface in a paper-absent condition is replaced with an electric signal by a reflector 3 and stored in the RAM. Then, a printing paper is inserted, stopped in the paper thickness detecting device part, and held between a detecting member and a platen 1. The paper thickness sensor 8 is pressed, an electric output E2 is detected and stored in the RAM and, therein, the paper thickness is calculated according to the electric outputs E1 and E2. Then, a platen gap suitable for the paper thickness is set. Thereafter, the carriage is shifted by the amount of the total digit number, and the reflector 3 detects a reflectance on a body to be detected different from that of the platen surface is detected, whereby a paper width is calculated and a printing start position and a printing area are determined.