PURPOSE: To prevent such a state that a printing head is held pressed to a platen when a power source is cut off by reducing the wt. and volume of a carriage, by using a shape memory alloy as the material of the spring for pressing the printing head to the platen at the time of printing operation.

CONSTITUTION: When a current is supplied to a pusher spring 5 made of a unidimensional shape memory alloy by the input of a printing order, said spring 5 generates heat and develops its shape memory effect at certain definite temp. or higher to be ready to return to the stored shape. This force becomes larger than the tensile force of a release spring 4 to press a printing head 1 to a platen 11. When the input of the printing order is eliminated, the supply of a current to the spring 5 is stopped and the generation of heat is stopped and the temp. of the spring 5 is lowered. At this time, the spring 5 becomes a stretched state by the release spring 4 and the printing head 1 is separated from the platen 11. The temp. of the spring 5 also lowers to the ambient temp. when the power source of a printer is cut off during printing operation and the printing head 1 is returned by the release spring 4 to be separated from the platen 11.