PURPOSE: To provide a residual amount detecting device where the residual amount of the sheet to be unwound as a feed roll is rotated is correctly measured and displayed, the constitution of parts is simple, and the arrangement space can be suppressed minimum.

CONSTITUTION: A photosensor 57 detects the rotational angle of a feed roll 24, and a rotary encoder 56 detects the unwound amount of the ink ribbon which is unwound as the feed roll 24 is rotated. The central processing unit (CPU) computes the diameter (r) of the feed roll 24 by the unwound amount 1 of the ink ribbon per rotation of the feed roll 24. In addition, the CPU compares the computed diameter (r) with the set value r 1, and if (r) is not greater than r1, the diameter is outputted in a residual amount display part, which displays that the residual amount of the ink ribbon is small.