An impact printer has a platen (1), a carriage (5) movable in parallelism to the platen (1), a hammer (9) carried on the carriage (5), driving means for protruding the hammer (9) toward the platen (1), detecting means (11,12) for detecting the speed of the hammer (9) striking against the platen (1) and returned therefrom, and control means for changing over the speed at which the hammer (9) is protruded toward the platen (1) in conformity with the result of the detection of the detecting means (11,12).