PURPOSE: To speed up the printing with a stable operation by changing the running speed of dots in linkage with the driving of a printing magnet for varying the cycle of excitation intervals corresponding to the thickness of papers.

CONSTITUTION: After the loading of a dot printer with a paper 9, a motor 13 is driven to rotate a pole screw 14 connected to the motor 13. A base 15 advances to move a detector on a table to an ink ribbon 8 and the paper 9 of a printer. As a paper thickness sensor 16 is pressed against a platen 10 with the movement of the detector, an arm 17 of the sensor 16 is pushed back and the rear thereof 17 is separated, from magnet 19 with a shaft 18 as fulcrum pressing a microwitch 20. A signal from themicroswitch 20 stops the motor 13 and simultaneously, the base 15 is halted. If a position indicating section 22 is provided at the bottom of a guide 21 sliding through the pole screw 14 on the undersurfce of the base 15, the position of the base can be indicated with a position detector 24 provided on a motor mounting base 23.