PURPOSE: To enable to obtain an optimum printing density without generating any excessive heat at a driving element, by driving a hammer magnet by a logical product of a driving signal for a printing hammer and a row of printing density controlling pulses, in an impact printer.

CONSTITUTION: When selection signals are selected in accordance with the thickness or the number of sheets of a printing paper used in a line printer, for example, when IMP1 is selected to be L while IMP2 is selected to be H, an inverted output of the raw of density controlling pulses generated by an oscillator OSC1 is outputted from an NAND circuit (which is connected to OSC1) through an AND circuit A, and a transistor Qn is turned ON or OFF by the logical product of the inverted output and a hammer-driving pulse (a), whereby an amount of printing energy according to the selection signals is given to the hammer. In this case, when both of the selection signals IMP1, IMP2 are selected to be L, outputs from the NAND circuit and the AND circuit A become H, and the hammer performs a printing action with the maximum energy. Accordingly, the optimum printing density can be obtained without generating any excess heat from the driving element.