Abstract: A carton sealer is provided whereby a bent-back portion having non-sticky outer surfaces can be formed at a tip of an adhesive tape that is supplied in order to join flaps on a carton. The carton sealer of the invention can join flaps on a carton that is conveyed by adhesive tape. The carton sealer comprises: a tape sticking mechanism that applies adhesive tape from an adhesive tape supply unit onto a sticking-on region extending from a lead region on a front face of an advancing carton, through a joining region extending in the conveying direction, to a tail region on the rear face of the carton, a cutter for cutting the rear end of the adhesive tape that has been applied, and a suction head that forms a bent-back portion at the head of the following tape formed by cutting. Reduced pressure subjected to the suction head acts on and applies suction to the non-sticky surface of the adhesive tape, thereby bending the adhesive tape back so as to form a bent-back portion having non-sticky outer surfaces.