PURPOSE: To provide a transfer paper size detection device which can prevent a contact failure between the elastic piece of a size detection terminal for detecting the size of a transfer paper and a size distinguishing pattern.

CONSTITUTION: A size detection terminal 6 in which a plurality of elastic pieces are formed slides on a size distinguishing pattern formed on a base board 35 in accordance with the sliding movement of a width setting plate 32 (32a, 32b) slidably provided in order to detect the width size of a transfer paper fed from a paper feeding part 10 and detects the width size of the transfer paper by the position in which a plurality of elastic pieces are brought into contact with the size distinguishing pattern. Since the shape of the size distinguishing pattern with which the plural elastic pieces respectively are brought into contact by the sliding movement thereof respectively differs, the number of times at which the elastic pieces are deformed at the time of the sliding movement thereof, differs and since the elastic pieces having the larger deformation are the easier to bring about fatigue deformation and consequently the easier to bring about a contact failure, the elastic pieces are made into such a different shape for every elastic piece that the elastic piece having the larger deformation brings about the harder contact failure, thereby being angle to prevent a contact failure.