PURPOSE: To prevent impact sounds from leaking to the exterior and enable heat to be released after printing, by providing doors for closing vent holes when a printing head is moved.

CONSTITUTION: When the printing head 9 is moved in a moving range (a) during printing, the movement is not transmitted to a power-transmitting lever 8, and the doors 2, 3 remains closing the vent holes to maintain a casing 1 in a hermetically sealed state, so that the impact sounds generated by the head 9 do not leak out to the exterior. In the cases where cooling is required because the head 9 or a circuit element reaches a high temperature or where printing data are not supplied, the head 9 is moved to the left beyond the printing ranage (a), whereby the lever 8 is pressed, and the force is transmitted through wires 4, 5, 6, 7 to open the doors 2, 3, and ventilation of the casing 1 is conducted.