A blank feeder is disclosed having a magazine with a delivery opening at one end and accommodating flat blanks as arranged from the one end toward the other end thereof in the form of a stack. The blank feeder also has a transport device having a path of transport extending straight from the delivery opening as a starting end to a terminal end away from the opening, and a device for unfolding each of the flat blanks to a tubular form of square cross section while the blank is being transported by the transport device. The unfolding device has first and second blank unfolding claws provided at edge portions of the delivery opening so as to be individually engageable with opposite ends of the blank in the delivery opening. The first and second unfolding claws are positioned at different distances from a reference plane orthogonal to the transport path in a direction along the path.