A dot line printer in which, as its head carriage is driven, a plurality of printing heads arranged on the head carriage at equal intervals are moved in a direction perpendicular to the sheet feeding direction, to print on a printing sheet set on the platen. The dot line printer has a linear scale secured to the head carriage, and a linear sensor provided on a head carriage supporting frame, and a sensor for detecting the displacement of the linear scale to produce a detection signal which is processed to obtain the present position of the head carriage and the correct printing timing of the printing heads.