A knife folding machine comprises: a support surface (4) for supporting a lower surface of a sheet (S); a knife blade (5); a pair of folding rollers (6a, 6b) opposed to the knife blade (5) at a fold position with the support surface (4) therebetween. The knife blade (5) is reciprocated between first and second positions by a knife drive unit 7. The first position is away from an upper surface of the support surface, and the second position is adjacent a gap between the folding rollers. The reciprocal movement of the knife blade (5) effects a folding operation every time the sheet (S) is set at the fold position. The sheet (S) is conveyed to the fold position by the conveyer belt (13). The conveyer belt (13) is driven by a first servomotor (11). The control unit (12) controls the rotation of the first servomotor (11) so as to decelerate the sheet (S) before its abutment against the stopper (14) without its rebounding against the stopper (14).