The printing machine for variable format offset includes a printing roller or drum, and at least one printing group including a guide system fixed in relation with the printing roller or drum, a first carriage displaceable along the guide system and configured for supporting a rubber-wrapped blanket roller or sleeve of variable size, a second carriage displaceable along the guide system and configured for supporting a plate roller or sleeve of variable size, and a third carriage displaceable along the guide system carrying an offset inking head. The head has devices for approximating the third carriage to the plate roller, identifying and fixing its working position in relation with the plate sleeve and fitting ink-supplying roller or rollers to the plate roller.