A linerless label roll-up preventing device for a thermal printer that prevents roll-up of the linerless label 3 on the platen roller 13. A roll-up preventing plate 23 is applied to a slit (22) formed in the outer surface of the platen roller (13), and protrudes to form a roll-up preventing edge (29) at a portion of the roll-up prevention plate (23). A roll-up preventing plate 23 being fixed to a slit (22), formed in the radial surface of the platen roller (13), relatively to the platen roller (13), and formed with a roll-up preventing projection edge, at a portion of the roll-up preventing plate (23), projecting out slightly from the outer circumferential surface of the platen roller (13) and capable of contacting the back surface of the linerless label (3).