PURPOSE: To even the printing density of a printer to decrease the printing sound and to lessen the load of a printer head, etc., by providing a detecting means for detecting the amount of a gap between a platen and the printer head and automatically correcting the amount of the gap.

CONSTITUTION: In this dot printer apparatus, a displacement sensor 14 of a semiconductor strain gauge or the like is provided to detect a gap between a platen and a printing head. An output signal of the displacement sensor 14 is input to a timer 16 through an A/D converter 15. The timer 16 has an input of a printing trigger from a printer head driving circuit 25, measuring the time interval when the output of the displacement sensor 14 shows its peak and outputting the interval to an operating circuit 19. The operating circuit obtains the optimum value of a platen cap from the distance data read out from a data ROM 18 and the time interval, drives a platen cap motor 23 based on the obtained control signal, thereby to maintain the gap between the platen and the printing head at the optimum value.