The machine, which is equipped with a print head (5) which is movable vertically between two walls (25, 26) of its housing, comprises a device intended to prevent said motion, when the character selection rod (20) of said print head is left in its extended position.The selection rod comprises an engagement element (23) for the engagement of a series of rotary elements (16, 17) which bear printing characters.The safety device comprises a retractable movable stop (28) firmly connected to the print head (5) and subjected to the pressure of a spring (35), and a fixed counter-abutment (29) rigidly connected to a wall (26) of the housing.At the end of the return stroke of the selection rod (20), the movable stop (28) is retracted by the push of said rod, whose engagement element (23) then develops a retaining force which is greater than the action of the spring (35).