A box blank holding device for a box-making machine has a main body, a driving rack and a motor mechanism for reciprocally moving the driving rack relative to the main body. A transmission shaft is rotatably supported on the main body, the transmission shaft having a pair of pinions, and one of the pinions being in meshing engagement with the driving rack. A pair of holding-member actuating racks are disposed in meshing engagement with the other pinion. Each of the holding-member actuating racks has a holding member pivotally mounted thereto on an end thereof. Each of the holding members also has a roller thereon. The rollers are guided in respective guide grooves provided on the main body for guiding the holding members between projecting and retracted positions for holding and releasing a stack of box blanks. By using a fluid cylinder as the motor mechanism, the holding force on a stack of box blanks can be predetermined.