Abstract: A method of assembling a paper box, comprising a 1st step of carrying a paper box (10a) with unfolded both side wall members (26) and (27) into a clearance between a fixed stand (101) and a guide member (100) in the condition that the positions of folded parts (22) and (23) aligned with the positions of both outer end parts of the guide member (100) at a folding station (47) where the guide member (100) is disposed with a slight clearance provided above the fixed stand (101) and a clearance at both end parts thereof set equal to a clearance at the folded parts (22) and (23), a 2nd step of driving a folding member (105) so as to fold both side wall members (26) and (27) to the inside with reference to the outer end part of the guide member (100) in contact with the folded parts (22) and (23), and a 3rd step of extracting the paper box (10a) with bent both side wall members (26) and (27) from the folding station (47) by holding the front side of the paper box (10a) in its moving direction.