Abstract: In a platen gap adjustment device, a stable area detection device for a platen gap formed between ahead and an upper surface of a platen, wherein the carriage guide shaft is moved relatively to the platen so that the platen gap is adjusted by driving the drive motor to rotate the gap adjuster cam, the gap adjuster cam is configured so as to provide a plurality of stable areas and a plurality transition areas; and wherein a stable area detection sensor is provided so as to face to a rotational member which rotates synchronously with the gap adjuster cam, and a detection object in correspondence with the stable areas of the platen gap is provided on the rotational member.